First thing you can do is slap a "TM" on the design somewhere. Still means you want to protect it but it's not a REGISTERED trademark yet.
IANAL, but I think the real point in registering a trademark is that the government gives you the proof of who had it first in a dispute - you can say "I registered my trademark on such-and-such a date, when did you register yours?" Whoever registered first wins.
Just in case, might want to print out that logo, sign it, and mail it to yourself. That way, you've got a government postmark on there proving you at least had that design on that date. Not sure how much that'll stand up in court, and certainly isn't as good as an actual registered trademark.