What does a taught Master's admission board consider important?
So my question is: besides having strong letters of recommendation, what can I do to increase my chances?


Not much. You're applying right now, so there's no time to make your application look stronger.  However, in math at least, no prior research experience is expected for master's programs, or even most PhD programs in the US.  Your GPA looks fine to me.  I don't think you should have trouble getting into a good master's program in the US (in math or CS) provided you're coming from a quality school and have good letters.  You could also consider sending out some applications for PhD programs if that's what you want.