One thing about washing and fading I found out with a silkscreened t-shirt, which would probably also apply to acrylics is that there is a difference in harshness among detergents (and between detergents and soap). If you want to keep the colors as saturated as possible, don't use Tide (which is one of the strong ones), but something like either All or Cheer (can't remember which) which is much less harsh. (And do remember not to use fabric softener, as someone mentioned, because it will act as a resist in the long run... and also don't wash it for at least a week or two to give the acyrlics maximum time to bond.)
If you're going to do t-shirts as well as jeans, then a "watercolor" technique can be really nice too. Just use acrylics and thin them down with water (cheaper) or acrylic medium. In fact, I believe "fabric" paints are just thinned acrylics with medium already in them. Just thinning them makes them much less stiff as well. The colors will look delicate and ethereal.
One way to do that is to spray the cloth first to dampen it, then brush or drop paint on it which will cause it to spread and thin out (or you can spray the paint after it's on)... the wetter it is and the longer you leave it, the more it will spread and "watercolorize." It can be quite beautiful.
After it's dried, you can go back with a thin fabric marker (I think I used a black Pigma Micron pen?) and outline or draw over the colors ... the existing blobs of color can act as a general guide for the shapes (outlining losely just inside the colors is nice), or you can just draw whatever you want (even over a lightbox and pattern). Fortunately, it doesn't take a lot of drawing ability to make this look good
"Masking" can work really well with this technique as well ... I colored over some stained white t-shirts for the nephews with liquid dye or with thinned acrylics, both in spray bottles. I first put down some cardstock shapes or anything else (like steel bars or freezer paper --ironed on) which wouldn't curl easily when wet, and sprayed with one color around them and here and there (those spots will stay white if the masks are not removed till the end). Then I put on more shapes, and sprayed with another color, overlapping and also just spraying a bit all over .... etc. Repeat till it looks the way you want, then remove everything (or remove some things before spraying with the next color. Let dry... and don't wash for awhile. (The greater the height the spray comes from, the more even and light the coverage, but small splats can be good too for more interest ...toothbrush spraying, etc.).
Another fun thing I did to "mark" some old t-shirts that I'd decided were too ratty at the neck for the DH, was to "scribble" print and/or stamp them. For the first, I just scribbled acrylic paints of various colors or tones quickly onto a piece of aluminum foil ... then I turned the foil over and pressed it onto the shirt gently so it wouldn't smear. Great as a thing for kids to do too ...except for the smearing part, you can't really mess it up.
For stamping, I used any kind of spongy, wood, etc., shapes I had, with acyrlic paint.
Wood shapes can do a cool thing on paper... can't remember if I tried it on fabric though: put more than one color on the wood, press down and twist just a bit, then pull away from the fabric...leaves an interesting pattern. The DS and I did some of these for Valentines.
I also used SoftScrub or Cascade dishwashing gel (on stamps as well as freehand) on non-white fabrics ... after stamping, let dry and wash out. Works best on natural fabrics, as do many of the other techniques.
You can also screen print without needing a "real" screen by making a small screen (8x10, e.g.) yourself. It can be quite cheap.
Jean Ray Laury does this and has at least one fabulous book out on doing all kinds of inexpensive printing ....she's a very resourceful and creative person! and packs lots of things into that book. Let me see if I can find the name of it because I know she's written more than one:
Imagery on Fabric:
A Complete Surface Design Handbook
(this is actually a second edition with even more techniques ...boo hoo, I have the first edition . . . and here's one place which sells it and shows its Table of Contents:http://softexpressions.com/software/books/laury.html#sta
It's billed as a "quilting" book, but it's mostly about the tehcniques themselves and how to do them on fabric.
(Actually, I would buy ANY book from Jean just because she's so unstingy with info, tries to find the easiest and least expensive ways to do everything, and is an excellent and organized teacher.)
Wow, I got going on that one, didn't I