Do you mean as in construction wise or accessory wise? Though I don't change my tees, I do add. Adding a vest is really cute, or suspenders, if you can pull them off. A lang neclace with a pendant, or onlt that you loop twice around your neck is really cute also. Even if you just and something cute in your hair, as in a headband with a flower on it or a cute bow clip will amp it up a bit!

Do you mean as in construction wise or accessory wise? Though I don't change my tees, I do add. Adding a vest is really cute, or suspenders, if you can pull them off. A lang neclace with a pendant, or onlt that you loop twice around your neck is really cute also. Even if you just and something cute in your hair, as in a headband with a flower on it or a cute bow clip will amp it up a bit!

I have this great thing I do for over sized t-shirts. I cut off the bottom half just below the bust. THEN I take the bottom half (Just a loop now) and bring it in to fit your waist. Then, sew it back to the top being suer to bunch the extra fabric in teh top at the front. It makes a gret pesent looking shirt and for people with small chests (Like me!) It makes you look like you actually have a chest!!

do you have any idea what you want to do specifically?
i don't have a whole lot of experience, but i'm pretty good at figuring out how to do this type of thing to achieve what i want, so if you had a specific idea maybe i could help you get started with it?
& honestly when i started getting into diy clothing stuff, i just dove right in.
http://craftster.org is an amazing site to check out. there are a fair number of tutorials, and tons of people available to answer any questions you might have. and who knows, something on there might inspire you.
just have fun with it~

u said that u wear plain t-shirt...if so then u should wear with them something with plenty of things on it...

I embroider a lot of shirts, which is fun if you know how. Also, you can make shirts into other things: I just finished making an old wifebeater into a vest, and right now I am making a skirt from a different shirt.
Go to Value Village and get some $1 shirts, and experiment. Once you feel comfortable with your techniques, start on your real clothes.

Another thing to do is cut strips on each side of a tight shirt, it's really cool (literally, too)!

And of course, don't forget pants. You can be simple and make them into shorts, embroider, cut the sides and sew them back together with a ribbon. I've even made a purse from some old jeans, and it looks awesome! Everyone keeps asking me where I bought it.
You can make holes in your jeans by yourself, it's much cheaper then the pre-ripped ones, and looks much cooler :P To do this, you can use a cheese grater, or what i prefer to use, is one of those little tools that take out bad stitches.. it's the best way to keep the white strings, and not have a big hole.

HAVE FUN CREATING!

If you're really wild, you can try a Gwen Stefani-before-she-sold-out sort of look.
Take an old shirt, and cut it about 2-4 inches below where your bra would end. It looks great if you have decent abs.
You can also buy one of those net-shirts, and do the same.