You would be surprised how many errors could be caught and corrected by comparing the answer to a rough estimate to see if it makes sense! Practice using the plotter to get a True Course from a sectional by making up arbitrary routes between points and measuring the distance and course between them until you are comfortable that you can get the correct information quickly and accurately. Note the course and distance information on a notepad, you will need to figure out where your top of climb and top of descent points will be along your flight path.
When traveling cross country, we make our plan to fly at certain altitudes depending on the terrain we need to pass over and, in the case of a VFR flight, the VFR cruising altitude requirements for easterly courses odd altitudes plus 500 feet or westerly courses even altitudes plus 500 feet when above 3000 feet above the ground AGL. Altitudes on maps, and in Area Forecasts and Winds Aloft Forecasts are given as height above mean sea level MSL and you will plan to fly your route at a MSL altitude. Altitudes for cloud layers in Terminal Forecasts and METARS are given in distance above ground level or AGL, so you must add the airport elevation to AGL altitude to get the cloud level relative to MSL.
A scrupulous pilot always considers where they would land in the event of an engine failure or other major problem. According to the Pilot Information Manual for a Cessna 172N, the plane will descend at about 675’ per mile at best glide speed, for example descending from 8000’ to 4000’ in 6 miles. Ask yourself as you pick your route, could I glide to some place where I could land and survive from any point along my route? If not, you may want to consider other options!
The regulations specify that one must fly at least 1000’ over a congested area, 500’ over a ‘non-congested area’ and at least 500 feet away from any person, vehicle, vessel or structure in sparsely populated areas. When planning a cross country trip it makes good sense to adopt the more conservative IFR terrain clearance standards of at least 1000’ above the ground and 2000’ above mountainous areas. When it is windy and you are flying over mountains an even greater clearance is desirable, both to get further away from turbulence caused by the winds flowing over the terrain irregularities and as an extra margin of space between yourself and the ground in the event that strong downdrafts are encountered.