In Gaia Ventures Ltd v Abbeygate Helical (Leisure Plaza) Ltd  EWHC 118 (Ch);  PLSCS 18 the court looked again at the meaning of these much used (and perhaps) much misunderstood terms. The case concerned development agreements in Milton Keynes and a dispute over an overage payment.
In reaching a decision in favour of the claimant for £1.4m, the court explained that in giving a meaning to the term "reasonable endeavours" it must make a value judgment and do so in light of the particulars facts of the particular case. As such, simply because doing something might be adverse to your own self interest does not mean it should not be done. In addition, if you agree to use "reasonable endeavors" to achieve an outcome you should not do something that makes this less likely.
As for "best endeavours" this is a much more onerous obligation leaving little room for value judgments to be made about whether or not a step is should be taken. This is especially so if you agree to use "best endeavours" to achieve a defined outcome.
Even so, the Gaia Ventures case is a nice reminder that even by agreeing to use "reasonable endeavours" you cannot simply refuse to do something because it might not be in your own self interest. In this case, failing to abide by that obligation cost the defendant £1.4m.
an obligation to use reasonable endeavours to achieve a stated aim requires a party to take one reasonable course, not all of them, whereas an obligation to use best endeavours probably requires a party to take all reasonable courses he can. A promise to use “reasonable endeavours” or to take “reasonable steps” is not to be interpreted as a promise to act “if and to the extent that it is in conformity with my proposed arrangements”. The question is whether a relevant step is feasible, and then whether in all the circumstances it is reasonable to take it (or unreasonable not to take it), balancing the risk of adverse consequences against the obligation to perform.