This is an important question, with two simple answers.

If you are a beginner or just looking to have fun, don't invest any more than is 'fun' to lose. Hence, if you're comfortable blowing a hundred bucks, put in $100 and see if you can win with it. This is what I did. My original roll was only $100, but I built it up into my current, much more powerful bankroll.

Bankroll considerations are different for an experienced player who has proven himself or herself a winner. These types of players are looking to consistently make money at a given limit. If you are one of these players, you should be able to bank 200 big bets at the limit you play. Hence, if you play $1-$2, you should have an $400 roll. For $5-$10, your target roll should be $2,000. These numbers prevent you from blowing your entire bankroll because of one bad run.

Some say that 200 big bets is too few for shorthand, but I believe that you need to be reasonable about potential losses. You don't want to invest more than 200 big bets unless you've proven that you're successful at that limit.

Also, it is a very good idea to keep a daily diary of your sessions. This will help you determine if you are a winning or a losing player and how often you win. It will also prove helpful come tax time if you live in a country with income tax on gambling winnings (in the UK and Canada, there is no tax on gambling winnings).

Good Luck from Gataville!!!