Estimating Time and Resources

In IT we are often asked to estimate the time and resources assorted tasks will take. Often these time/cost estimates are tied to funding, grants, and resource allocations. Unfortunately, many of us struggle at coming up with estimates more accurate than "it will take longer than expected". What are some strategies and best practices we can use to come up with better estimates? Under what circumstances does estimating things become easier? Harder? Under what conditions should we spend a lot of effort making estimates, and under what circumstances should we not?

When have you had good experiences making estimates? When have you struggled?

As always, bring your experiences and questions. Also, please spread the word about this meetup so that more people who do nonprofit systems administration will become aware of it.