The best advice I can give is to tailor a proposal to an individual company. This can take a lot of work, of course. But it's easiest for me to evaluate an artist's work if he produces something appropriate to one of my lines -- for example, if his sample is in a retro-D&D style appropriate to the Dungeon Crawl Classics, or if he provides mech drawings to show he can draw DragonMech. One or two picture samples via e-mail are usually fine (though send a URL, not the art itself).
I occasionally get submissions from people who clearly have no idea what Goodman Games publishes (unless they think I have plans to publish political comics, or anime titles, or other things that obviously aren't part of my lineup). Those are the ones I reject quickly, simply because it's obvious they're not that interested in the product. You've gotta be interested in the product!
Other random bits of advice:
- Don't e-mail huge attachments; a URL is best.
- Meeting in person at a convention really isn't any better than e-mailing submissions (at least when it comes to me). I meet soooo many people at every convention. I come home with a huge stack of cards, and it can be hard to keep all the new faces straight. Cons are a good way to try out the market, but I personally think an artist is better off collecting cards at a con, then e-mailing samples later (as in, a MONTH later, after the exhibitors have time to decompress and follow up on everything from the con).
- Most important piece of advice: Make sure your art is up to professional standards. I sometimes receive submissions from people who obviously have above-average artistic ability, but they're still not up to professional standards. Flip open a book from the company you're submitting to, and ask yourself, "Can I draw that well?" (And since I personally took art lessons for most of my life up to college, and happen to be one of those good-for-an-amateur-but-not-up-to-pro-standards artists myself, I always ask myself, "Can I draw better than this guy?" If so, he's not up to the standards!)
As it stands now, I have a good group of artists who are already helping me out, so I don't need any additional assistance. But I'm always willing to review portfolios and keep things on file in case I need help in the future.
Hope that helps.