One thing I've noticed recently is just how many climbers don't do any form of getting warmed up again after resting between harder routes. Maybe I'm weird, but for me it's vital to do a short 're-warm-up' before trying any hard redpoint or onsight, and I know many climbers who feel the same. The rest between attempts might be anything from belaying your friend for 30 minutes to taking half a day off before another big burn on a long route, and the optimum length of any re-warm-up will usually be proportional to the rest. This re-warm-up serves to get the fingers and arms ready for hard moves again, to help avoid injuring yourself by trying hard moves from cold and to get some blood flowing to avoid getting flash-pumped - all the same things your inital warm-up is there to achieve.
If it's been an hour or more, I'll usually start with a short walk or jog to wake my body up, especially if it's a cold day. Ideally the crag will have an area where you can do a bit of traversing and a few boulder problems, if possible on the style of hold that your route involves. Unfortunately most crags wont be as perfect as this (unless you spend a lot of time at raven tor or rubicon!), but you can usually find some holds to hang and do chin-ups on, as if you were warming-up on a fingerboard. If this doesn't work, branches in trees can provide a good alternative, and you can always drop a finger or two off on small branches to replicate pockets. Alternatively, climbing the first couple of bolts worth of your chosen route can work well too, presuming they aren't the crux of the route or on tweaky holds.