वर्तस्व च सतां क्रमे

न शक्यसे वारयितुं गच्छेदानीं रघूत्तम।
शीघ्रं च विनिवर्तस्व वर्तस्व च सतां क्रमे॥ २-२५-२॥
–श्रीमद्वाल्मीकिरामायणे अयोध्याकाण्डे पञ्चविंशतितमे सर्गे
na śakyase vārayituṁ gacchedānīṁ raghūttama |
śīghraṁ ca vinivartasva vartasva ca satāṁ krame || 2-25-2||
–śrīmadvālmīkirāmāyaṇe ayodhyākāṇḍe pañcaviṁśatitame sarge

It is not possible to restrain your departure [any longer]. Depart now, Oh best amongst the Raghus!
Return soon, and tread in the footsteps of the righteous!”

Amazing! The first advice the great Kausalya gave Her Son, is to tread in the footsteps of the righteous! Surely, Lord Rama didn’t need that advice (of course, it’s for us), but Kausalya just underlines the importance of that. No wonder, this is also one of the most fundamental cornerstones of Sanatana Dharma (and the need for a Guru!). It’s also worth quoting another fantastic conversation in Mahabharata (thanks to the eloquent Vasu Srinivasan):

कः पन्थाः ? asks yakSha. “What is the road?” I would have just asked back “To where?”.

तर्क: अप्रतिष्ट: श्रुतयो विभिन्ना: न एको ऋषि: यस्य मतम् प्रमाणम् ।
धर्मस्य तत्त्वं निहितम् गुहायाम् महाजनो येन गतः स पन्थाः ॥

Yudhishtra’s first utterance is “tarkaH apratiShTaH”. A quote on which vyAkhyAna can be done for hours I guess. “Logic is baseless”. Logic has been the considered the greatest employment of human intellect since Aristotle, Plato down to the European philosophers, who have spent their lifetime only in logic. But Yudhishtra dismisses it curtly – Logic is limited, baseless and cannot be relied upon.

“srutayo vibhinnAH” – vedA-s say different things! “na eko rShiH yasya matam pramANam” – There is not a single rishi whose word is an authority! Its just a poetic way of expressing that vedA-s are interpreted differently and every rishi worth his beard has an opinion. “dharmasya tattvam nihitam guhAyAm”. The truth about dharma is hidden in a deep cave! What the Huh? Where did dharma come from? What does it have to do with logic or vedas or roads? Each rishi is attempting to explain what is “dharma”, but nobody has a single opinion about it and nobody really knows what dharma is. And then he finally ties them all beautifully – “mahAjano yena gataH saH panThaH” – the road travelled by great people is the one to follow (for mokSha)! The great ones have already figured it out, you just follow the road paved by them. yakSha was not even asking about a physical road! That is the fault of translation or misunderstanding of contexts.

See also:

षडेते नित्यदुःखिताः

ईर्षुर्धृणी नसन्तुष्टः क्रोधनः नित्यशङ्कितः।
परभग्योपजीवी च षडेते नित्यदुःखिताः॥
īrṣurdhṛṇī nasantuṣṭaḥ krodhanaḥ nityaśaṅkitaḥ|
parabhagyopajīvī ca ṣaḍete nityaduḥkhitāḥ||
The envious, the malicious, the angry, the one who is ever-suspicious,
and those who thrive on others’ fortunes – these six are always miserable.
See also:

अपृष्टस्तस्य तद्ब्रूयात्

शुभं वा यदि वा पापं द्वेष्यं वा यदि वा प्रियम्।
अपृष्टस्तस्य तद्ब्रूयाद्यस्य नेच्छेत् पराभवम्॥५-३४-०४॥
–विदुरनीतिः श्रीमद्महाभारते उद्योगपर्वणि चतुर्त्रिंशोऽध्यायः
śubhaṁ vā yadi vā pāpaṁ dveṣyaṁ vā yadi vā priyam|
apṛṣṭastasya tadbrūyādyasya necchet parābhavam||5-34-04||
–viduranītiḥ (śrīmadmahābhārate udyogaparvaṇi caturtriṁśo’dhyāyaḥ)

Be it good or bad, or pleasant or unpleasant.
Unasked should that [truth] be spoken to the person whose downfall is not wished
i.e. The well-wishers of a person should always advise the person unasked, and tell them the truth, whether or not its agreeable!

See also

चक्रवत् परिवर्तन्ते

सुखमापतितं सेव्यं दुःखमापतितं यथा।
चक्रवत् परिवर्तन्ते दुःखानि च सुखानि च॥
sukhamāpatitaṁ sevyaṁ duḥkhamāpatitaṁ yathā|
cakravat parivartante duḥkhāni ca sukhāni ca||

A similar saying:

जयोऽस्तु पाण्डुपुत्राणाम्

जयोऽस्तु पाण्डुपुत्राणां येषां पक्षे जनार्दनः।
यतः कृष्णस्ततो धर्मो यतो धर्मस्ततो जयः॥
jayo’stu pāṇḍuputrāṇāṁ yeṣāṁ pakṣe janārdanaḥ |
yataḥ kṛṣṇastato dharmo yato dharmastato jayaḥ ||
Success be to the sons of Pāṇḍu (Pāṇḍavās), on whose side is Janārdana.
Where Krishna is, there is dharma and where dharma is, there is victory.

[Thanks to sanskritdocuments.org]

भावोपहतानि कल्कः

तपो न कल्कोऽध्यननं न कल्कः स्वाभाविको वेदविधिर्न कल्कः।
प्रसह्य वित्ताहरणं न कल्कस्तान्येव भावोपहतानि कल्कः॥
— श्रीमद्महाभारते आदिपर्वणि
tapo na kalko’dhyananaṁ na kalkaḥ svābhāviko vedavidhirna kalkaḥ|
prasahya vittāharaṇaṁ na kalkastānyeva bhāvopahatāni kalkaḥ||
— śrīmadmahābhārate ādiparvaṇi

Tapa is innocent, study is harmless, the ordinance of the Vedas prescribed for all the tribes (varNas?) are harmless, the acquisition of wealth by exertion is harmless; But when they are abused in their practices it is then that they become sources of evil.
(Translation by Kisari Mohan Ganguli)

नैव राज्यं न राजाऽऽसीत्

नैव राज्यं न राजाऽऽसीत् न दण्डो न च दाण्डिकः।
धर्मेणैव प्रजाः सर्वा रक्षन्ति च परस्परम्॥१२-५९-१४॥
—महाभारते शान्तिपर्वणि ५९-१४
naiva rājyaṁ na rājā”sīt na daṇḍo na ca dāṇḍikaḥ|
dharmeṇaiva prajāḥ sarvā rakṣanti ca parasparam||12-59-14||
—mahābhārate śāntiparvaṇi 59-14
[At first] there was no sovereignty, no king, no chastisement, and no chastiser.
All men used to protect one another righteously, by dharma.
This may be construed as the description of an ideal kingdom. Bhīṣma said this to Yudhiṣṭhira when he [Yudhiṣṭhira] inquired about how kings and kingship came into being. This is verily the description of the Kṛta age, where sovereignty first began.
Ganguli translation of Mahābhārata, Chapter 12 (Śānti Parva)

सत्यादपि हितं भवेत्

सत्यस्य वचनं श्रेयः सत्यादपि हितं भवेत्।
यद्भूतहितमत्यन्तम् एतत्सत्यं मतं मम॥१२-३१६-१३॥
—महाभारते शान्तिपर्वणि ३१६-१३
satyasya vacanaṁ śreyaḥ satyādapi hitaṁ bhavet|
yadbhūtahitamatyantam etatsatyaṁ mataṁ mama||12-316-13||
—mahābhārate śāntiparvaṇi 316-13
It is always proper to speak the truth. It is better again to speak what is beneficial than to speak what is true. I hold that that is truth which is fraught with the greatest benefit in all creatures (This is said by Devarṣi Nārada).

The saying “satyādapi hitaṁ bhavet” is frequently misunderstood. The scriptures do not say that truth should be sacrificed in view of what is beneficial, for such view will militate with the saying that there is nothing higher than truth. The saying has reference to those exceptional instances where truth becomes a source of positive harm. The story of the Rishi who spoke the truth respecting the place where certain travellers lay concealed, when questioned by certain robbers who were for killing the travellers, is an instance to the point. The goldsmith’s son who died with a falsehood on his lips for allowing his lawful prince to escape from the hands of his pursuers did a meritorious act of loyalty. Then, again, the germ of the utilitarian theory may be detected in the second line of this verse (from Reference 2).

Also see:
[1] http://www.sacred-texts.com/hin/mbs/mbs12316.htm
[2] http://www.sacred-texts.com/hin/m12/m12c029.htm

यतो धर्मस्ततो जयः

जयोऽस्तु पाण्डुपुत्राणां येषां पक्षे जनार्दनः।
यतः कृष्णस्ततो धर्मो यतो धर्मस्ततो जयः॥
jayo’stu pāṇḍuputrāṇāṁ yeṣāṁ pakṣe janārdanaḥ |
yataḥ kṛṣṇastato dharmo yato dharmastato jayaḥ ||
Success be to the sons of Pandu (Pandavas), on whose side is
Janaardana. Where Krishna is, there is justice and where
justice is, there is victory.

यन्नेहास्ति न तत् क्वचित्

यदिहास्ति तदन्यत्र।
यन्नेहास्ति न तत् क्वचित्॥
yadihāsti tadanyatra|
yannehāsti na tat kvacit||

"What is in here may be found elsewhere;
but what is not here can not be found anywhere else!"

This was proclaimed by Veda-Vyasa after composing the Mahabharata. Such is the comprehensive coverage of various aspects of human life in it! The Mahabharata is indeed called the fifth Veda (पञ्चमो वेदः) (see here). Find a nice brief on the great epic here.