The best advice I can give is to break down a task into smaller pieces then estimate those. Maybe attach a "confidence" percent to each sub-task to calculate an optimistic and pessimistic range. That way when your boss says "5-8 days? That doesn't help me at all, where are you coming up with these numbers!?!?" you just show him your breakdown and you have justification. A competent manager should put your butt on the line for what you commit to, not what you digress to be bullied into committing to.
