I wish this column at Snooth wasn't in slide show form, but it's a pretty interesting discussion of what causes and remedies there are for wine going bad when you can't finish a bottle... with this as the take home message:
1. Minimize surface areaBy which he means, if you know you're not going to drink a whole bottle of wine transfer what you plan not to drink it to a smaller container (minimizing surface area and exposure to air) and put it in the fridge (cooling it). Anna use the Wine Saver which the author seems to think isn't a great solution... his argument being you're sucking all the good aromatics out with the air... but we've had surprisingly good results with it... at least in the respect of the wine lasting well beyond when you'd think it'd be vinegar.
2. Keep it cool
3. Introduce as little oxygen into the wine as possible