Dont sell your domains when time is not right. Domaining is a speculative business,like real estate, waiting and guessing is part of the game. A pro would tell you that each domain should be sold to the right buyer at the right time to get maximum results.
Once you have had a good appraisal or you know that your name will sell for great price, your next step is to evaluate offers you recieve carefully.
Reply to each offer you recieve with your counter offer, carefully explaining your domain worth or ROI. Once the buyer knows that you are not going to settle for less he may choose to come close to your selling price.
Another example of profitable waiting would be if a new TLD is introduced and you have got a top 1000 name. In such cases wait for 2-5 years till the TLD becomes popular and prices start to go up.