This is what I do if I really would like an upgrade.

It also depends on the hotel brand type as well. For non-FS hotels (i.e. CY, FFI, etc.) I tend to just ask about an upgrade when I check-in. Most of these are not really resort properties and don't really have much to upgrade you to. The exception is with RIs. I now call the Plat line to make all my RI reservation (award and paid). While making the reservation, I will ask the agent to call the hotel and ask about them upgrading me to a 2 bedroom suite. (Yes, I know suites are guaranteed, but some RIs and CYs will still hand them out if you ask.) I've had good luck in getting pre-upgraded at RIs into 2 bedroom suites. I've very rarely been pre-upgraded to a suite room at CYs, but I'm running about 80% (4 of last 5 stays) with getting a suite upgrade by asking while I am checking-in.

At FS hotels, I will do the same thing that FlyinHawaiian does. I will normally send an email about a month or so before my stay to the GM. More often than not, the GM will forward my email to someone in their reservations department, but almost always I will get a pretty good "pre-arrival" upgrade. Note I don't do this often, maybe once or twice a year at most, as I save this for the times I really want a better upgrade.