If your Mugo pine is already overgrown and larger than you want, it is probably too late to correct that. Mugo pines do not adapt well to severe corrective pruning. Typically, pruning stimulates dormant growth buds along the stem allowing you to trim and reshape the plant sit your tastes. However, new growth on a Mugo pine only occurs on the tips of the stems, so if you prune along the stem, those cuts will result in stubs and a disfigured appearance. But, you can still take steps to slow down growth with a special pruning technique specific to pines.
Remove any dead or weak branches with pruners. Sometimes pest infestation or winter damage has disfigured the shrub. Cut these branches all the way back to healthy growth.
Monitor the plant during spring to determine when the new growth has fully emerged. but has not opened; this new growth is called a candle. A candle is a brown, thin, tight cluster of needles that grow upright from the end of a stem, usually in bunches of three. They vaguely resemble a candlestick, hence the name.
Cut the candles in half, straight across. You can use pruners or scissors. The new growth is soft and is easy to cut.
Fertilise the plant lightly with a general-purpose fertiliser formulated for non-flowering evergreens.
If you want a Mugo pine to stay a limited size, it is important to purchase one that is grown specifically for its dwarf habit. Research the various cultivars available and check the tags carefully at time of purchase. Unfortunately, purchasing a Mugo pine for its size is not an easy task. Michael Dirr, renowned plantsman, says this about Mugo pines: "Perhaps one of the most confusing pines for the homeowner because of tremendous variability in size. The cute, diminutive prostrate plant that comes from the garden centre and is placed in the foundation planting often becomes 10 to 15 feet tall and wide although it was advertised as a 2- to 4-foot high low-growing evergreen."