Some folks have success with using 'Image' to get some control on wild onions, depending on your grass type of course. Here is a link to the msds labels, check first to see the recommendations based on your grass .http://www.amdro.com/Image/labels.html
They are an ornery adversary because they have a waxy covering that makes it difficult to break down and penetrate. You can try mowing them before the application to cut the top, but be sure to spray immediately after.
You can use a pre-emergent on your flower beds to help prevent weeds from sprouting. Also a nice layer of mulch down to suppress the germination of weeds in the beds. I have found that most of the winter weeds in bed are pretty easy to hand pull actually if you have your soil mix right with the much layer added. Depending on the size of the bed this may not be feasible for you though. I like to weed, yeah...call me strange....lol...lots of 'dirt time' is good for the soul :)
For the onions, these can be rather annoying and somewhat labor intensive to get rid of. Your first choice is simply to dig them out, roots and all. Also, something like Roundup will kill them, but you have to break the leaves/stalks of the onion plant and sort of "wash" it with the Roundup. I have heard (though not tested myself) that mowing your lawn every 5 - 6 days for a few weeks will also kill them off.
Also, Travis P posted something about the Image stuff 4 Seasons speaks about here: http://www.hometalk.com/activity/113614
Mike, best solution (and good exercise) is from my days as a Technical Director of a major housing complex. Get some ground turf made specifically for the purpose of preventing weeds and install...pretty easy. This stuff lets water in but does not let weeds grow through. You could also do what my Mom does and use garbage bags, just be sure to leave about an half inch space around flowers and shrubs to allow them water.
Mike, is it an annual flower bed or one with perennials? If there's nothing in the bed you want to come back in the spring, you could cover the whole thing with a thick layer of newspaper or cardboard, cover that with a layer of compost or good mulch and then water the whole thing well. By spring the cardboard or newspaper should have broken down and you can till everything into the ground.
I am having the same problem, I have been pulling the onions, but cant seem to get ahead of the weeds. Mine are perennial beds. I think the warm weather we are having is contributing to the weed problem.
Carole, I do a late-fall (November) application of a pre-emergent (Preen in my case) to help with just the situation you describe.