I'll speak for myself, but also add a tip or two that I've tried to take on board...
Match breathing with steps. As in, breathe in for three steps then out for three steps (or every four steps or whatever depending on slope). It's the regular pattern that matters because it helps keep breathing - and therefore heart rate - under control.
If hands are free, place a hand at the top of each thigh and push down to assist the legs as they straighten. Or clasp hands behind the back. Both methods help keep body upright and assist breathing.
Every so often, take a very deep breath and blow it out hard. This helps clear the lungs for a good intake of fresh air.
Avoid panting at all costs!
(I see that Richard has replied while I've been typing... absolutely, I like an energy gel (or other boost) when approaching a tough climb. I think it helps, but I'll admit the benefit might be psychological!).
(I see that jimbob has replied while I've been typing the above... yes, I agree that shortening the stride and taking baby steps can help. However, for long, tough inclines, I like to vary my stride and foot strike where possible.)