There are several ways in which the troposphere can become unstable. The atmosphere can become thermodynamically unstable by solar heating of the earth's surface. Once this heating reaches a certain threshold, surface air parcels can rise, condense into clouds and develop rain-laden clouds. If high pressure exists aloft, then only expect fair weather cumulus on a day with strong surface heating.
Adding moisture to the low levels of the troposphere makes the troposphere more unstable. Moist air is less dense than dry air making it more buoyant than dry air at the same temperature. Air with a high moisture content and saturation will condense more precipitation as it rises. Low level instability can be rapidly brought about by warm air advection along with moisture advection (bringing of a warm and humid airmass into the lower troposphere).
Cooling the middle and upper levels of the troposphere causes the troposphere to become more unstable. If the middle and upper levels cool, the potential temperature of the low levels of the troposphere will be relatively warmer. Warm air underneath cool air is an unstable situation (especially when the cooling with height is large; cooling with height much greater than normal). To produce clouds you simply need two ingredients: rising air and saturated air. Air can rise through these processes: fronts, orographic lifting, meso-scale circulations, upper level divergence, solar heating and surface convergence. A mid-latitude cyclone has many of the previously mentioned rising trigger mechanisms (low level convergence, upper level divergence, fronts). Strong speed and directional wind shear can enhance convection. Examples include: Jet streaks, vorticity, low level jet, low level horizontal vorticity.