Things go wrong sometimes.
Keep it simple.
If you're fortunate enough to have regular training sessions between games, sessions where the goalkeeper's specific needs are addressed, then this post won't be of much use to you. Maybe you even have a qualified goalkeeper coach to work with. I was once in that position, long ago, before the internet and mobile phones (but not before Ryan Giggs). These days, though, I'm like many other goalkeepers who have to turn up to play once a week without having had a proper training session since the last game.
As the first line says, the first rule of thumb is to keep things simple. If you're deciding between hoofing away a back pass or taking an extra touch and finding a teammate, this while being closed down by an opposition striker, just hoof that mother away. If your instincts tell you a winger on the byline is going to cross, meaning you can maybe lean away from your near post a bit, think again. If a swerving shot has you unsure whether to catch or parry, parry - especially if you haven't caught a football in a week.
Of course, these are just guidelines. Each situation is different, and should be approached as unique. The general point is to recognize the limitations that come with not playing. There is also a difference between dealing with a swerving shot in the 5th minute as opposed to the 75th minute. Later in the game, you're going to feel more comfortable, less rusty, and perhaps won't have to take into account your lack of training. But in the early minutes, when you're feeling your way back into goalkeeping, don't make life harder than it need be.
Dammit! I forgot to be good.
Regular training makes the simple things - routine handling, positioning, little collapse dives - more or less automatic. In absence of this, we have to think our way through the game a bit more, remember to consciously order our feet to move, our body to back up the hands.
One thing I like to do is use my pregame warmup as a mini training session. The only difference to a regular warmup is that I try to do it at a faster pace with more repetition. If I can take thirty shots into my hands and ten to fifteen crosses, I'll be that much more likely to handle them correctly once the game starts.
One final idea: if you have a football around the house and you've got nothing better to do, pick it up every now and then, bounce it a bit, handle it. If you have a solid wall and understanding neighbors, play a game of catch. Tennis balls are good for this, too. If you happen to break anything, this sentence serves to absolve me of blame. :)
As usual, these are just my thoughts. I'd love to hear yours - how do you stay match-fit when you don't get to train?