How to get higher credit limit

I'm sort of a complete noob when it comes to how credit cards work, even though I've been using them for over 10 years now. My credit score hovers around 700, few points above or few points below and I feel credit limit on all my cards is pretty low. I have around a $5k credit limit on each of my 4 cards.
My practice has been paying off each credit card immediately after buying something, so I pretty much always have a zero balance on them.
I'm wondering, how can I get higher credit limits like $20k+ and how to improve my credit score? I feel like I've been stuck in the 700 score loop for years.
Thanks in advance for every reply.