Some tips in no particular order:

The thing people don't brag about when they celebrate their RPs' 1, 2, 3+ year anniversaries are the 95%+ of their [i]other[/i] RPs that died in the first month. It's a death-heavy hobby, if you can't cope with that then you might be better off writing solo. I've been at this for over 7 years and do you know how many of the RPs I've been in lasted a year or more? [b]Three.[/b] And of those, only [b]one[/b] ever got an actual ending. We're in this because it's fun while it lasts, not because we think every RP will last forever. 

[b]Reputation is  e x t r e m e l y  important.[/b] If you're new and don't have much posting history for people to look at (or if you mainly RP in PMs), write up some writing samples (or some previous RP posts you're particularly proud of) in a Gallery thread for potential partners or GMs to peruse. If you have a history of making an ass of yourself in the OOC, you'll have a hard time. Et cetera. 

[b]Honestly evaluate your skill level and write at that level.[/b] Not everyone is cut out for Advanced or even High Casual, and that's okay. Write with people of a similar skill level (no shame in dipping into Free if that's where you fit), be receptive to criticism and take an interest in improving your writing and you WILL get better, and in turn you'll have more options open to you. Because yes, the quality of your writing often DOES matter to potential partners/GMs. You might not like it, but it's true; and luckily, it's entirely within your control to change.  

[b]Most of all, be willing to put in the work.[/b] Focus on making every post better than your last and over time you will get results. Better yet, your partners and group members will recognize the effort you put in, and hopefully appreciate you for it. What WON'T get you results, though, is giving up because you think you're doomed or that nobody "understands." [b]Life isn't fair[/b], nothing worth doing is easy, you don't always get rewarded for your efforts, and the only good way to cope with it is to keep trying anyway.