Transition defense is the hardest defense to develop and the Suns exploit that brilliantly. If somehow you can get players to play good transition defense and and slow the game down, you'll win. A lot easier said than done. The Spurs were really good at that two years ago.
Take a note from the Lakers game plan from the playoffs last year. It was a good game plan and despite the Lakers losing (to the better team), they took them to 7 games.
You can't do that to them as much this season though, because Amare and Kurt are back. They make it a little harder than just T2 and Diaw.
The way I've noticed to be effective against the Suns is:
#1. Shoot a high % from the field. This helps keep the Suns of the fast break. Be patient and run your offense. Don't always take the first open shot from the outside. Make the extra pass, and make them expend energy on defense.
#2. Make sure one man runs back on defense after a shot is attempted. The Suns (especially Marion) like to release after a shot attempt and it gets them easy shots on the other end.
#3. Play some sort of zone defense. You can't do it to them over the course of a whole game because eventually their shooters are going to get you, but mix it up, collapse on Amare and force him to be a playmaker.
#4. Don't get suckered into playing at their pace. This is the most important thing. Too many teams run with the Suns, and right now, no one does it better.
I think a big thing that made the Lakers so successful is the Odom/Marion matchup. Marion gets so many fastbreak points normally, and it sparks the offense and the team, and gives them big momentum. Odom was constantly backing Marion down and tiring him out, and basically keeping Shawn underneath the basket while he was on defense, to prevent those easy fastbreak points.
It seemed to be really effective, although with Amare back, it makes it all the harder.