Somethings require a big chunk of time now and then others you should do daily, but for all time is the main thing you need to spend.
1.Writing good titles and descriptions of your items - this can take about 5-10 minutes per item. If you haven't really thought about this before, you may want to set aside time to fix existing items. You want titles that buyers can search for and enough detail to narrow the field. Artfire has some great articles on this SEO 101 and and Improve your SEO. These are quick easy guides to get started.
2. Take good pictures of your items Online, buyers can pick up or touch your item, so you need to give them a really good look. Be sure to use good lighting and use the macro setting to get a crisp shot of details. Take pictures from all angles, close up and full shots. Be sure the main image is square so it the thumbnail is not cut off. Cropping and adjust pics is important, you don't need to invest in software, one good site for editing pictures is Aviary.com
3. Go to the Artfire forums - Even if you only take the time to read the Staff Announcements, you can keep up to date to on what's going on on Artfire. Things are listed there before the weekly email. I'd suggest posting, even if just in the item of the day thread. Your items will then show up on the right hand bar. Other sellers will get to know you and you can ask questions, learn tips.
4. Tweet a few times a day - I don't mean just tweet your shop items, tweet about your craft, your day, your thought, respond to other tweet. And occasionally add shop items and of course post anything you blog
5. Set up a Facebook Fan Page It takes about 5 minutes to set up, if you have a shop avatar, use that. Suggest it to friends, tell people about it to get fans. You only need 25 fans to set up a person URL. If nothing else, post links to your new blogs posts and add pictures of new work. A fan page post shows up on your followers' page.
6. Blog at least once or twice a week - If you haven't set up a blog, now's the time. Pick a focus, make it something that interests you. Of course have your Rapid Cart or links to your store, facebook and twitter. People will learn more about you and you'll get your name out there but you need to provide new posts on a regular basis
These are five things you really need to invest sometime into. Many are overlapping, like tweeting and posting what you blog. Some are one time things like writing good descriptions and taking good pics. If you haven't been doing any of these, start with 1. and 2. and work your way down.