Avoid pruning in the spring and autumn. It is during these periods that the maple is at its most active. During the spring the sap is rising to provide nourishment for new shoots. If you do have to prune, then seal off any large cuts to prevent the loss of sap. In the autumn the tree is flowering and is preparing seeds for propagation.
During the summer some trees can become overgrown with foliage and it may become necessary to trim this back. Usually it is enough to remove some of the inner foliage to allow for a free flow of air through the tree and to be able to appreciate the tree structure. Be careful not to cut too much and expose areas of the trunk to excessive sunlight that wouldn't normally be exposed.
You should also check the tree late winter but before any new shoots appear. Just trim away any dead twigs and check over the trunk and branches for any damage. It is at this time you would make any major trimming like removing lower branches.
If you decide to remove any major offshoots then always cut with the tree and not into it. Seal off any large cuts to prevent infection. You can use normal window putty for this. Avoid trimming branches opposite each other on the trunk and avoid trimming off branches directly below or above each other. This will help to avoid placing undue stress on the tree.
Trying to restrict the height of your tree by pruning will only damage the overall appearance and put the tree under stress. If your tree is growing too high then either you should move it or think about replacing it. The natural growth pattern for any tree is upwards.
If ever you are in doubt it is always worthwhile consulting with an expert who can advise you of the best course of action. Usually the best advice for anyone who is considering pruning a Japanese maple is to imagine you are cutting your own fingernails, only cut what is necessary.