How your boss assesses your work depends not only your on actual results, but also on whether these results are above or below the boss’ expectations. It is not how high you jump but whether you clear the hurdle, so lower the hurdle. Lower the expectations of the relevant party or parties: your boss, his boss, your subordinates, your peers, or partners.
It is usually a mistake to promise too much. Give people a pleasant surprise and give yourself a margin of error. Use the following tactics (within limits) and vary the chosen tactic
Say it will take longer. This is often believable because people frequently experience delays.
Ask for a lower target. One technique is to work out what resources it would take – people, money, etc. Often the resources cannot be fully obtained so the target has to be reduced.
Break down the target into parts and prioritize them. You will succeed in at least some parts. This process also has the benefit of clarifying expectations.
Be clear what a successful outcome would look like, and that it is the same for you and the other party.
Caveat: This technique only works if the task is not something many other people are doing. It is not going to work if there are performance benchmarks.