The other day I decided to calculate 1.225 x 36 in my head (this was a calculation in one of my problem sets) and I found that it took me a lot longer than I originally thought. So I was wondering if there was any sort of strategy to gradually improve this skill. Here's a strategy I am beginning to use: Before I go to sleep, I come up with with 7 problems (written down) to do in my head (simple ones like 23 + 43 and 346 - 987) and then check the answers with a basic calculator. Then, with my eyes closed, I do one hard problem (e.g. 3423 x 1490) so I can actually fall asleep. I've gotten considerably better with only a few weeks of doing this. But are there other strategies? Thanks.