Well with speed comes power, so I'm not too sure what you mean when you say "not stronger". Maybe you are talking about increasing your ability to launch a kick? At any rate, here are some things that I use to help my speed.
Consistency first and foremost. Whatever methods you choose to use, consistency is the most important aspect.
Stretching. Being flexible will allow you to shorten your range of motion in a given technique. It will also give you more opportunites to use your kicks.
Jump rope. This helps your rhythm and coordination. It also helps you to be light on your toes.
Form kicking. Practice your kicks on an imaginary target in the air. Make sure you use your best form, and really concentrate on your beginning and ending positions.
Tension kicking. Contract your muscles while performing a slow motion kick. Start with 10-15sec count kick. Its an excellent workout for the muscles needed in any given kick. Once you become proficient at that, you can add ankle weights to the excercise to further strengthen the muscles.
Weight training. Excercise the major and minor muscle groups of those used in your given kick. This will give you stength as well as speed.
These are just a few things that will help, and many more exist. Whatever you choose, make sure those things become habitual... and not occasional. Because it takes continual practice to become fast, continual practice to stay fast, and continual practice to get faster. Good luck with your training!
:asian: