Unless it is a very small hill, it is likely that everyone will slow to some extent. One thing you can try on smaller, shorter hills is standing for a few pedal strokes to keep your speed up, but please be careful when doing this. Practice pulling the bike forward as you stand so that you don't shoot the bike back into the person behind you. As the others said, on steeper hills the paceline generally breaks up, with everyone climbing in their own comfort zones.

To get the paceline to really work you need to get everyone to agree that they will not get frustrated with the idea that they aren't putting out their top effort at every second - that's the whole idea behind a paceline, you get to rest a while, then you take your turn. Keep the pulls short (30 seconds!) and the people who want to pull will be less likely to get bored and want to pull ahead when its not their turn. When you really get it going good everyone will be able to go faster - further and it will be fun!