You can get a deeper stretch by contracting your muscles once you settle in as deeply as possible, holding for a couple of seconds, then releasing. So if you're working on side splits, go as low as you can, then try to pull your legs together (WITHOUT actually moving- all you want to do is tighten your muscles). Keep contracting for like 10 seconds, then when you release you should be able to sink a little lower. Do this 3 times per stretch, it's less effective each successive time you do it.
Also, be careful when using this technique. You will be pushing the limits of your flexibility, and you can easily pull a muscle if you overdo it. At first don't do this more than once or twice a week (you'll probably be too sore to anyway). And DO NOT work out afterwards, if you throw a high kick when you're that loose you will injure yourself.
When I was doing this once a week I managed to go from not being able to fully extend my knees (I'm genetically very tight) to within a couple inches of a full split in a 4 months or so.