Standard advice is to maintain a cool, constant temperature for the first few days, then gently let it warm. If things seem to slow down earlier than usual, let it ride (often times, the last part of fermentation isn't as visible as the early times). If you really are stuck, rousing the yeast into suspension at a warmer temperature can help. Very last resort would be to make up a yeast starter, and pitch it at its most active (don't do this unless you're in a really bad spot).