My LBS (er, LBM - it's just one guy plus his assistant) stocked helmets in three sizes: XL, M/L, and S/M. XL was way too big. S/M was way too small. M/L, although looking good, doesn't seem to fit right. Even with the thick pads, and the buckles tight, I could push it over my head.

He had one helmet of a different make which fit better. But I can still get it off my head, with a little work. I bought it anyway, because I needed something to get started. But, as you can understand, I'm not happy about it long term.

Can anyone help me out? I don't need a fancy or aerodynamic helmet - but I do want one that will provide protection. I'm not sure where I should turn next.

It sounds like you need a helmet that's just "M".

Find another shop with more variety. I have a big head so it took me a few helmet try-ons before finding a good fit so I made sure to go to a big shop when I got my current helmet (Bell Ghisallo '04, IIRC).

What brand and model are you talking about?
Figure your circumference

and then find the correct helmet.

All my helmets are Giro Eclipse with Roc Loc 3. It keeps the helmet securely on my head.

Its not just the size but the shape of your head/helmet that must fit.
I use a MET which has a complete band of flexible (and adjustable length) plastic which goes around my head. It will fit just about any shape.

So is the only answer to find another LBS?

So it would seem- While I like the styling on a couple of the Giro helmets, the only helmet 'form' that seem to fitmy melon is Bell's. Their helmets fit me like a glove (nearly) so that's what I buy. I really, realy dislike an ill-fitting helmet, even more than a silly looking helmet. I've stopped even looking at other helmets.

Head shape is a funny thing- way variable, plus the size thing on top. Find out which manufacturer's helmets fit you best (by shopping other LBS'- try on everything, even the high priced helmets you'll never buy, then you'll know what a better fitting helmet feels like) and then you will likely be able to shop price online for the next purchase.

My most recent helmet is a Trek, now with added Ziptite. That is, it has an adjustable band to keep the helmet on the head. It works pretty well, but we'll see how it does in a year or two. Giro, Trek, Bell all have helmets with adjustable fitting systems. If you wait for a sale at store, you should be able to find a pretty inexpensive helmet that fits. I got this one for $20.