Zak123, there are a lot of good suggestion here and I'll add one more beside the practice of quessing and walking it off until you get better at it.

Estimate the distance half way, than double it.

Most often your furthest target will not be more than 40 or 50 yards on the long side. Therefore if you can get a good grip on 20 or 25 yards you can estimate out to fifty with no problem.

I have a range finder for hunting but I continually practice estimating as I good for walks. I estimate the distance to the next tree, street sign or when inside the end of a hall or the next store in the mall. It is like anything else, practice pratice practice.