Based on my own experience:
1- Hold the punch which is going to be struck in a leg vice or if you use a regular vice attached to a table, put an extra leg under it. The idea is to make a firm connection to the ground to avoid bouncing. I use an extra Ikea metal leg under my vice.
2- Put a piece of pure led under the punch to be to be struck. It will absorb the shock and reduce bouncing. Pure lead can be found in fishing stores.
3- Use heavy enough hammers. A 1,5 kilos hammer is usually enough but on occasions it can be a little light (it all dependes on the size of the counterpunch you are about to use). I also have a 3 kilos small sledge hammer.
4- Don't temper your counterpunches or temper them just very lightly, otherwise they will deform.
5- Protect your eyes. Steel fragments could fly at high speed and injure you.
6- In split counterpunches, like the ones needed for a or e, pay a lot of attention to the design and depth of the slit because it will make the middle stroke. After some frustrating experiences using gravers or a little saw I found it is easier to use a simple counter counter punch similar to a chisel to cut the slit in the split counter punch.
7- Never strike with a counter punch a letter or design that has already be cut. You need thick walls and mass around the crater you will produce. Otherwise the strike of the counterpunch will fracture the steel walls and ruin the letter.
8- You can't used counterpunches in big punches because you would need an enormous force (well, except if the counterpunch needed is much smaller than the punch itself). In such cases you could heat the punch to be struck with a torch until it's red and strike quick with a counterpunch. This is adviced – I think – in Moxon but it's a dangerous operation and obviously not for the newbie. I prefer to just dig big counters with gravers.
9- Use pure lead for testing the shape of counter punches. Type metal is too brittle and it will shatter if you try to strike it.