I was always way under with my estimates. One day I was talking to some senior dev at another company and he said that he got better by multiplying his estimates by 3.

So I gave that a go, I'd do my estimates in one column then multiply them by 3 in the next column. At first I thought that those x3 estimates were way too high but 90% of the time they were about right, maybe a little over but it's usually better to be over than under.

After a while I just started landing on those x3 estimates without it being an extra step, I've been not bad on the estimation side of things since doing that little trick.