I used to sail the 420 a bit - couple of key differences (some basic, some not basic):
1. Huge centerboard + rudder compared to a laser, don't get sloppy with bearing away etc, just because you can do it brute force style doesn't mean you should!
2. Sail to the telltales!
3. Play the main to keep it flat rather than pinching - you don't have the issue with staying at VMG angle because the jib always tells you (in the laser when you dump the main you lose the guidance of the telltales). You also don't have to worry about the rig changing shape when you ease the main
4. Kicker - pull on until the top telltale collapses behind the main 70% of the time.
5. You need to have a good set of rake/spreader/chock/tension settings - set up spreaders/rake for expected conditions, but ALWAYS compensate by adding/removing chocks + adjusting rig tension as the conditions change. You've got a lot of flexibility in the way you configure the rig, so if it's going to be howling and you're on the "depower" settings your sails will be flat as a pancake regardless of the small adjustments you can make with cunningham/outhaul etc. If it then gets light, adjusting outhaul + cunningham WON'T give you enough, you need to change rig tension and put chocks in etc to get to a "best fit" setting with your current mast setup.
Does that make sense?
6. Goes over waves REAL nice compared to a laser - keep that rudder in the middle and let the boat ride out the waves all by itself while you concentrate on VMG and sail trim. It should not be a physical boat to sail - if anything other than your mainsheet arm is getting worn out they either your unfit or you're trying to sail it like a laser!
Blocks aren't an option on the mast. I wouldn't necessarily even use a tension gage- make sure the rope part of the jib halyard is something slippery, so you can do a 3:1 or 5:1 purchase with the cleat. Rig settings are so easy with the club version- light air is enough to keep the mast from wobbling, medium air is tensioned by one person, and heavy air is one person tensioning with the crew pulling on the forestay. Then adjust the shroud pins for rake (get a guide for that) and off you go.
pay attention to boat trim- you can steer a lot more with the boat/hull. The main can never be too tight. Get an adjustable bridle as well, I know there's a post somewhere on SA about it, or I could post a couple of pictures of mine. APS sells one too.