I recently went from active to the reserves. My new employer offers a 401K with matching up to a point. I'm contributing up to that point, but I'd also like to put some of my reserve pay in my TSP. My question is, how do I make sure I'm putting in the most I can without going over IRS limits? Each system has mechanisms to make sure I don't go over but that only works if you're only contributing to that system.

Also, what happens if I do go over accidentally?


You'll definitely want to track it yourself and make sure you don't exceed the deferral limit ($18K in 2015). If you do go over, you'll have to contact one plan or the other and request the "excess deferral" and associated earnings be removed. Here's a link to a fact sheet on the subject--check out page 4.  Good luck!