It starts with weather. A pilot knows where he wants to go VFR and the first step is to see if the weather will allow a flight along the route that, at this point, is just in his mind.
The most direct route is the first one considered. The next thing is the weather synopsis, or, the weather map. One word here. The TV weather maps paint with a broad brush and usually try to cover the weather for a whole day. The aviation weather prog charts, as shown on aviationweather.noaa.gov/progs/, are for specific times, out 48 hours. Why is it important to look at these? If there is a front or a low-pressure area affecting the route, a pilot needs to know about that. If you are flying toward a front or low, odds are the weather will get worse as you fly along.
The area forecasts give general information about the weather and the airmets will offer prognostications about IFR and mountain obscuration, turbulence, and icing. They are, respectively, named Sierra, Tango and Zulu. More severe conditions are covered in sigmets and convective sigmets for thunderstorms.
Most of us, though, use the terminal forecasts as the main tool in looking ahead for expected weather. These do have to be taken with a grain of salt because they cover only the weather around the airport. Only the area forecast covers the whole area and should be studied for every flight.
The available weather information has to be merged with the proposed route to get the real picture. If rough terrain is a factor, the height of the terrain has to be considered in relation to the forecast ceilings. The mountain obscuration airmet addresses this, but most pilots have learned that the broader forecasts, such as the area and the airmets, often paint with a broad brush.
The metars, the surface observations, also have to be considered and compared with the forecasts. Is the actual weather better, the same or worse than forecast? These reports of actual weather are worth a lot when planning a VFR flight.
For IFR in flat terrain the minimum en route altitude gives 1,000 feet of clearance, while in mountainous terrain it is 2,000 feet. Those are good planning altitudes for VFR as well. For example, if you want to head out across the eastern mountains, just look at the higher terrain within five miles of a proposed route and add 2,000 to that for a minimum acceptable ceiling.
Some will say that is too restrictive, but the whole deal about planning a VFR flight is to plan one where you can both fly visually and maintain a reasonable altitude above the highest obstacles along the way. That gives some margin because everyone knows that while the weather can be better than forecast, it can also be worse than forecast.
Often, for a VFR flight of some length, an alternate route might be a better weather deal. There are flights where some miles out of the way will keep the airplane over lower terrain, or farther away from a low-pressure area, or away from an area where thunderstorms are forecast, or along a route where there are more airports to use if the weather is worse than forecast. Certainly the most direct route is not always the best route.
The Nexrad picture of precipitation is as important to the VFR pilot as it is to the IFR pilot. While reasonable VFR conditions can exist in rain, odds are they won't last long. Lower clouds tend to form in rain areas, and the rain itself obscures visibility and makes it more difficult to see and avoid those lower clouds. If there is a widespread area of rain, chances of making it through there VFR are none too good.