What is the best penny slot strategy with $20?

I don't know if you guys have a strategy, when playing penny slot machines or not but there is what I do like to hear your strategy. I start with $20, If the machine I am playing is not hot, I continue playing it for the whole $20, if I get the free game bonus, I cash out and move to another machine. If the machine is hot, I play for as long as its hot once it stopped, I try 5 more pulls and then cash out. Also I always start with 2 credits per line and do not increase unless the machine is hot then I increase to 5 credits per line.