A general "rule of thumb" on how many rubber bands to use is at least 2 bands for each pound that the model weighs. I.E., a five pound trainer would require 5 bands on each side of the wing for a total of 10 bands. A seven pound model would require 7 bands on each side of the wing for a total of 14 bands, etc. The last 2 bands should be criscrossed in an X pattern not straight front to rear, as this will hold the other bands from comming off in flight. I have also found it handy to criscross the first 2 bands as this seams to help keep the wing on square as you add the other bands. Rubber bands CAN be reused once or twice IF you clean them so the the exhaust residue/fuel/oil dosen't attack the rubber. I keep a small coffee can half filled with kitty litter and throw the bands in as I remove them and shake the can so they are buried in the kitty litter. This will soak up the oil/fuel mess and the band can be used more than once. If in doubt, THROW IT OUT!!! Two or three days use max, then use new bands. Use common sense and caution.