Lots of good courses on Coursera and edX. Khan Academy is good too. I particularly recommend the A-level prep sequence from Imperial College London on edX

But if you really want to maintain and maybe even further develop your math skills after getting back up to speed, I think the best long term strategy is to do personal creative and/or commercial projects in domains that interest you and that make heavy use of math. E.g. low level 3D graphics programming, etc