There is some excellent information about this in The Pragmatic Programmer. They advise that you use appropriate units of time rather than estimating 130 days estimate 6months. They also advise to concentrate tasks that are most crucial. And avoid making estimates based on sub estimates.
Personally I find it is useful to break the task down to understandable chunks to properly estimate them. If the task is large there are too many nooks & crannies that can hide unthought-of problems. By concentrating on the details of smaller chunks, you can evaluate the potential problems more successfully.
