So this is a small collection of links that, in no specific order, are GREAT resources for first-time and veteran roleplayers alike (Let's admit it, it's easy to get stuck in bad RP habits!). If you have something you'd like to add/contribute/etc, please share!
MMORPG RP Basics
RP basics for MMO environments. This is written from a WoW player's perspective, but everything they say is applicable in any environment. Be sure to check out the rest of the website and their survival guide, it's a great resource.
General Roleplaying Tips & Advice
Another great website with lots of great tips/RP etiquette. Check the links at the bottom of the page for a lot of good character development/character building tips.
How To Craft Compelling Characters
For writers of all genres and flavors. Don't be one-dimensional, it's boring! Give your character some substance, some depth, something to make us want to get to know them!
RP Character Playability Test
I know it's at the bottom of one website I already linked, but I felt that this deserved it's own space.
11 Ways To Be A Better Roleplayer
I don't necessarily agree with everything on this list, but they make compelling arguments for their reasoning. It's also geared towards tabletop roleplaying.