Lately we've been doing a lot of kicking into swimming for our pre-sets, main-sets, and finishing sets. Kicking, followed by swimming (continuously, or after a short interval rest) can be an effective way to teach an athlete how to hold a strong kick while swimming.
Here are a few ways I like to work with this concept:
1. Wall Kick. Have the athletes kick for 5-10 seconds on the wall (8 and under style), then on the coach's whistle the athletes must do a flipturn or an open turn into a fast 25 with a big kick. Swim back easy, and do 8x50 like that on (1:15). This is a great pre-set!
2. Very simply: go 3 rounds of 5x100 Kick-Swim. The Kick should be with a Freestlye with a Kickboard, and the swim should be Freestyle. Do 5 on (130), 5 on (125), and 5 on (120). Rest an extra 30 before the final 5.
You can do the same type of set with 200s as well. I wouldn't go too much higher than that in terms of volume, because you're not going to get the same driving kick.
Each of the first two items help specifically with an athlete's ability to kick effectively while swimming. They tend to get a nice amount of white water going on the kick part, and transition that white water to the swimming portion. It's got to be FAST!
3. Phantom Wall training. This can be all kicking, or a mix or kicking and swimming like you see here in this example by Erika Erndl (The Phantom wall exercise happens about 40 seconds into this video clip): http://www.youtube.com/watch?v=RnVoaVOdJgk
4. Flip it around and go Swim-Kick. 24x50 with every 4th fast (25 Swim + 25 Kick underwater) is a great way to do this. Take an extra 10-15 before the fast 50s if needed. Time both the total time as well as the underwater 25. Breaststrokers may swim breaststroke underwater instead if they like. The athletes should be challenged to make their underwater portion as fast or faster than the swim portion!