There are a number of things you can do to make your harmonica overblow better.
Most of these techniques are aimed at making the reed respond better to the player.
1. Gapping the reeds will affect the reed's response to pressure.
2. Arcing the reed will change the reeds behaviour when bent.
3. wax or nailpolish is used to stop squeeking reeds
4. Embossing narrows the slot to minimize leakage.
5. Tip scooping changes the airflow around the reed.
6. Corner rounding allows a reed to be gapped low without it sticking.
It must be said that almost all modifications to the harmonica that are aimed at improving the responsiveness of the reed will have a very clear side effect; the reeds will sound more brightly thatn they did before. This effect is most noticable in reeds that are embossed and is even a reason for some players to not use these modifications because they do'nt like the sound it produces.