As you get better at it it shouldn't be as pronounced but it's normal becasue the muscles are undergoing lots and lots of tiny contractions to hold you in place. Oh, I forgot to mention another good way to progress on planks is to go to one leg on the floor. That's a challenge.
Here is a page with some other plank stuff plus some good exercises. I've tried the "core row". I say an MMA fighter doing it..I forget who. Really cool. Also they have the exercise I call "barbell russian twist".
http://www.strengthcoach.com/public/1455.cfm
Another good exercise is the pallof press. This is an anti-rotation exercise involving a pulley machine. You prob can't do this but it you get yourself some stretch bands you can do this plus a lot of very good core stuff in general with bands. I just got extra bands so I could really go to town with core stuff.
Anyways the pallof press involves standing with one side facing a pulley (or band tied off at around chest height) and you hold the weighted pulley handle (or band) at chest height. The weight is going to want to rotate you toward it. So you are resisting it. Then to do a "rep" you bring it out in front of you (increasing the rotational force) and back in as if your are "rowing it". This is a 'counter rotation' movement and is one of the main functions of the core. You will love it.
Here is something from Tony Gentilcore's blog on it:
http://www.bostonherald.com/blogs/en...ld-be-doing-2/