Fertilize often; at least twice per month with liquid food. When your trees are weeded, the food will reach the roots of the tree, not the weeds
Check for insects, especially mites; use spray, or apply systemic insecticides
Check for fungus, especially on juniper and elms
Don't let your trees dry out -but don't overwater them either. Check them every day and water them when they need it.
Become a watering expert, this is more important than a pruning expert
All trees should be outside now. Be sure to put the pines and juniper and tropicals in the most amount of sun; maples and azaleas in filtered light. BUT protect freshly potted trees and indoor trees from too much sun too fast, avoid getting a sun burn on tender new growth
Pinch back pine candles and lightly prune leaves on the tips of deciduous trees
and last, not least: Come on out with your trees for our Get Wired Sundays