Ckangaroo - I have always tightened by props with 4 way wrenches when possible (some engines nuts are too large and do require the monkey wrench). I have been flying 2 strokes for a long time and I have never had props fly off. Although I don't deny that a spanner would be the best, it isn't necessary.

jnes - first I wouldn't use a wood prop if you're a beginner. Plastic is much more durable and easier to keep tight. Then, I would check the following.

1. Are you using the provided washer and nut? You need to make sure that the washer is used in conjunction with the nut.
2. Have you overtightened the spinner to the point where the self tapping screws are too loose and the spinner can be nearly pulled off with the fingers?
3. (As kingwoodbury said) Check for hydrolock by taking out the glow plug and pouring out excess fuel. You can check the plug by plugging it into your glow starter and seeing if it burns bright red.
4. Are you under he age of 10 so that you cannot apply enough force to secure the bolt (serious)
5. Are you starting the starter first then smashing the head into the spinner? Don't do this... it's not safe. Always turn the prop clockwise (by using the starter head like a finger) until you feel compression and then push the starter cone onto the spinner. Last press the starter button to flip it over.

I thought I'd have a lot more "checks" but that's about it.

"This happened with my second flight..." You're already flying this thing with the prop flying off? Honestly, I think you are just not tightening it enough.