In my experience most technical interviews were done on whiteboards. Therefore, the best way for me to practice leetcode was to write out the solutions by hand, just like you would in an interview. It forces you to think ahead about the problem.

My strategy was to do 2-3 problems per day on pen and paper, and spend a maximum of 1 hour on each problem. Then, after that hour, check to make sure I got the optimal solution, and if I didn't, then also copy that optimal solution down onto the paper, so it gets burned into my brain.

This strategy, applied over a long timeframe, was extremely successful for me.