Correct there really isn't much bankroll management if your under $20. Pretty much everything is a risk. But you can still follow sound rules to avoid risk of ruin. Try these rules, they work real well.

"1. Never ever get into a cash game or SNG tournament where the buy in is more than 5% of the entire bankroll that you have. You can break this rule in case your bankroll is very small and you are going to play a Sit and Go or cash game where the buy in is lower than $2.5

2. Don’t join a MTT (Multi Table Tournament) where the buy in is higher than 2% of your current bankroll. By not risking you will be able to keep a lot of your money intact. This rule applies unless the buy in at the tournament lower than $1.

3. In case you have managed to win at a cash game and your money that you have at the table represents more than 10% of your online poker bankroll, then you should cash out of that table the moment the blinds are going to reach you." Chris Ferguson