HANDS TO THE SKY
First of all, sitting down on any shoulder exercise that you can is the way to go. When you are standing your legs are helping you lift the weight. When you are seated, the work is all kept in your arms. Try it, Do a set of front/side raises standing and then try them sitting. You will find that sitting is a lot tougher.
So these are my FAVORITE type of "shoulder press" that I do. When you do a shoulder press, the other muscle used to help the shoulders are the triceps. So I like keeping one arm above my head during the whole move to keep it in my shoulders and take as much work out of the triceps as I can.
HOW TO: Start out by holding your dumbbells above your head. Keeping one DB there, lower the other until your arm comes to a 90* angle or just below. Then push it back up to the starting position. Then repeat for the other arm. Shoot for 20-24 total/10-12 on each arm. Do it, you'll LOVE it!