Definitely a cheap poncho!!! We always have umbrella strollers, so we can fold them up and tuck them under something. If you have a larger stroller, I would recommend the dollar ponchos from Wal-Mart! They work like a charm and you can shake them out to reuse or just toss them! No fuss!
If you have an actual stroller cover, you have to deal with a wet stroller cover that may make the kids really hot inside the stroller! Ponchos have a great hole in them (for the head!) that the kids can get air through (not that the others will suffocate them!). It's just been easier for us with the ponchos!