Martingale strategy can be used when playing Video Poker. I'm doing it from time to time, and it's fun. You should try it. As it is always with purely luck based games, there's no guarantee that you will not lose your entire balance following this strategy, but if you are ready to stop in time, or your entire balance is not that big, try it, starting from the lowest bet possible, of course.
Video poker is not a real poker.
Martingale is a guaranteed winning strategy only under two small conditions: 1 - there are no bet limits, 2 - you have an unlimited amount of money.
The longest losing streak on roughly 50/50 outcome that I know of, was recorded on Just-Dice and it was over 30 bets long (can remember the exact number).
That being said, I've heard about brick-and-mortar casinos not allowing players to use martingale. So maybe it was somewhat profitable when playing in short-term bursts.
Also, it would be interesting to see more replies regarding the question in the OP.
Nah, enough has been said already. I think we settled that using the Kelly criterion is the only logical way of deciding the stake other than making an emotional decision.
This topic went off the rails a long time ago
OP, please, 
This thread is my gift to bitcointalk's gambling community
Not locking it. Ever.