How do you get the most out of them?

First of all, you need to establish the difference between light or reduced foods that actually do what they say on the tin and those that don't. For example, a reduced-fat treat such as a bar of chocolate or a biscuit can be the worst thing you could possibly eat: you give in to temptation and decide to cosset yourself with something sweet and chocolatey, but the low-fat pale imitation won't satisfy you. 

Light and reduced products are only effective if they have a very similar taste to the 'original version' - in which case they're probably still highly calorific (eg crisps, sorbet) or if you use them instead of a 'full-fat' food you eat lots and lots of, such as dairy products, in which case it can make quite a difference if you go for a low-fat version. The most important thing is to use them as part of a controlled diet, to fool yourself into eating that little bit healthier but in normal quantities, as part of a balanced diet.