Making the bicep stronger will help for sure.
Make sure when you slap it you're not putting your whole arm into it and tensing/locking the muscles in your hand/arm for a split second.
That's my habit and the split second sqeezes are what seem to build up and trap lactic acid around my bicep a 30 second break clears it.
I find it especially prevelant when sitting, as I sling bass lower than seating position so leave them there, which normally cut good circulation to the bicwp with the body, which is why I stand up lots when practicing too
