Depending on the tree involved and the intended effect there are several tree pruning strategies. They are listed here from those requiring the least work, to the most work.
An all over trim in spring or summer: This is only really appropriate for some smaller formal trees, especially evergreens. This will need doing every year or two. For these smaller trees a long-handled hedge trimmer is a labour-saving option.
Pruning when dormant: Usually involves shortening side-branches all over the tree to make it smaller and more attractive. It lets in light and reduces the tree's vulnerability to wind damage and is an opportunity to remove diseased or damaged wood. To get a balanced result, work slowly and carefully and evaluate the effect of removing each branch. This is not suitable for trees prone to silver leaf.
Pollarding: In this extreme form of pruning, the entire head or crown is removed which can make most attractive small trees, although at the price of repeated pruning.
Crown lifting: Lifting the crown by removing lower branches will allow access for mowing, mulching and enjoying the shade cast by the tree.
Crown thinning: Thinning crowns to let in more light by removing some, usually up to 30 percent, of the branches and concentrating on dead or congested shoots is another strategy.It is very easy to spoil the appearance of the tree so this is best attempted in stages evaluating the effect before removing more.
If branches larger than the diameter of your wrist need to be removed or if there is a lot of work up ladders needed, it would be best to call in a professional arborist. Pollarding, crown lifting and crown thinning are also all best undertaken by a professional.
Also be aware if there is a Tree Preservation Orders (TPO) on the tree. The tree officer of your local council will be able to advise if the tree is covered by a TPO.