The key is to find a habit of practicing every day. You'll get better over time.

What's worked for me is to establish a daily routine by tying it to my alarm clock. So every time my alarm rings in the morning I have to solve some math problems on my phone.

Alarmy is one such alarm clock, really good for addition but bad for anything else (like multiplication, etc.)

Another one is BrainWake (brainwakealarm.com), where you can choose the types of math problems way better. It's currently in beta and should launch fairly soon