I think the first thing you have to do is to stop thinking of marketing as something you go out and do. Marketing needs to be woven into everything else you do because, as you put it, you only have limited time. This means you need to think about various activities you do as potentials for networking and marketing, and thus dual-purpose as many activities as possible.
Some quick and easy things are:
An email signature which markets your services.
Recreational tech stuff (slashdot, etc) with signatures that link back to your site. Same with other forums.
If you can take up blogging as a way of both helping customers and marketing yourself, even better.
The next part is to weave marketing into the rest of your customer interactions. For example, the open source development work I do on LedgerSMB is all effectively marketing, but it is also R&D. (See how easy that was?)
(Edit: In case you missed it, making sure I put the link to LedgerSMB in the post here is an example of this dual purpose. I am making a point, being specific about what I am doing, and now if you hadn't heard of LedgerSMB before, now you have. Maybe some of you use the open source software, and maybe even at some point refer business to me. But hey, if not, at least I get upvotes, right?)