I didn't do anything to "winterize" it, but we only had a few stretches of really cold weather (less than 20F) and the majority of the winter was between 20F and 50F.
Should I do anything before firing it up and mowing the lawn?
Two-Stroke or Four? If four, check the oil. Check the general condition, e.g. check that the fuel lines are secure and in good condition. If it is water-cooled (I don't think they usually are?), check the coolant level and condition of radiator/hoses. Take the plug out, clean and re-gap. Empty out any old fuel and put new in (If the fuel is only 6 months old you can probably mix some of the old in with the new, it won't have gone off completely yet)
Turn the engine over by hand a few times before trying to start it (from my experience of pull-start engines you normally have to try several times before they'll start anyway!). Mower engines are pretty rugged things in my experience, so I doubt you'll have any problems - the above are pretty much generic points for any engine that has been sat for a while...
Nothing bad about all the things Nick C suggested, but for a simple push mower I think it's overkill, I'd just check the oil, top off the gas, and pull the cord! :-) If it sounds like it has a misfire, then next time around I'd check the plug(s). The plugs can go a very long time without maintenance unless they get damaged/rusty (and I'd be inclined to replace rather than attempt to rehabilitate them anyways). If properly installed with anti-seize and dielectric, you shouldn't have problems with plugs getting stuck even after years of service.