I use Prismamarkers as well, but I don't have the brush tips, just the standard large and small tips. When I'm working in a larger area that could easily end up being streaky, I usually color in one direction and work as fast as I can. It's kind of like the marker soaks the paper a bit so I find it's best to get that base flat layer down as quickly as possible. And I don't pick up my marker much because if I do, it overlaps and makes a darker spot than if I just keep going and maybe cross over a bit. However, if you want to attain flat, one colored sections that are larger, markers just aren't the way to go. They always have gradients in them due to their overlapping and just how markers are laid down. I personally enjoy they're "imperfectness". I have seen markers used beautifully for art nouveau styled work, so just keep playing around with them. Also, certain colors lay down beautifully and then some others don't.
In regards to textures, I like to lay down a base color first then layer it up with more colors for the textures/shadows/etc. I like to work with very light or sheer colors so it builds. Also, I've found that colors like "violet mist" when layered, actually kind of suck color out of the base color, so I love how it reacts with other colors. "Cool grey 20%" sucks color out too. I used to use the "colorless blender" but I ended deciding that it didn't really cooperate with me, but I know some people that swear by that thing.
I've found it's a lot of trial and error and just finding what works for you in a way that you like. I really can't say that enough. I just scribble about with them and happened over things that I'm now somewhat dependent on so to speak. And working with a limited pallet does wonders as well. You really have to figure out ways to layer odd colors to make a color that you don't have. Doing that made me think about my markers in a completely new way while I was making chibis for a convention a little ago. Don't give up. I never thought I'd be coloring with markers the way I currently am.
Also, paper makes a HUUUGE difference. If I use them on basic printer paper, they bleed like crazy. I know a lot of people that love using them on smooth bristol, but sometimes I feel like that sucks my color in too fast and I have less optimal working time for blending and such. I personally LOVE the Canson XL mixed media.
http://www.utrechtart.com/Canson-XL-Mixed-Media-Paper-Pad--Spiral--60-heavyweight-98lb-160gsm-sheets-MP57470-i1009801.utrecht This paper holds every kind of media that I like working with great. I've been watercoloring in it lately and still love it. With markers, it really lets me lay down a lot of colors in concentrated spots without a lot of bleeding while still feeling really workable in a way that bristol doesn't feel for me.
Sharpies seem to always be streaky in my opinion. I love them, but I avoid them for large areas.
Best of luck! I hope something I rambled about helps.
