What's the proper way to muddle mint for a julep or mojito?
The classic mistake when making a Mojito or a Julep is to over muddle the mint. Pounding away at the mint will release so much flavour from it, that you won't taste any of the other ingredients. A perfect Mojito should comprise a balance of flavours. The other main constituents do not have a particularly strong flavour, so its very easy to swamp them with mint and end up with a glass of alcoholic toothpaste. Mojito = Rum, Mint and Lime. Julep's are a little more tolerant, due to the richer flavours of the other ingredients, but the same basic principles still apply.

Don't overdo the amount of mint you add. Leaves from one decent sprig of mint will do. Muddle the mint gently for about ten seconds. You want to bruise the leaves and release a little bit of flavour, but not grind them into a pulp.

Finishing off a Mojito properly is also essential. You only need a splash of soda. Adding more than a shot will just dilute the ingredients and destroy the subtle flavours. Most importantly, don't forget to taste the drink after you've made it, even if its for someone else! This is the best way to learn and improve your mixing and muddling techniques. It is also sometimes possible to rescue an imperfect drink (eg. by adding a touch of extra lime juice if its too sweet) if really necessary.