Preparation for flight is not something to be done in 5 minutes. However lots of the data which you are likely to need during flight is right there in the flight manual and some deep study of fuel burn, passenger loading -v- CG and with that in your head the other day to day stuff like NOTAMS and weather can only be done on the day.
For me the most difficult task is sorting the weather. It is something that you just have to see from the aviators perspective and work at it. Weather and weather related problems are a continuous concern during flight. So you need to be well up with understanding what the TAFs and METARs are saying.
For my part I've done all the CG calculations for the machine that I fly and will know what is and is not possible. I know the fuel burn and it's effects on payload and CG. The wind velocity will determine the flight time which will demand a certain fuel load or an en-route fuel stop (check NOTAM for that airfield - Is it PPR - phone if needed - is fuel there - if I havent been there before get out the approach and landing data [Bottlang /Pooleys] - radio frequencies - nav aids, redraw the lines on my map to see if there are conflictions with special airspace/restrictions) and so on. I use a GPS but only after having done my planning on the map.
There is no easy way to flight planning, but the more you do it thoroughly, the easier it gets. If you get it 100% every time then you've achieved that through effort.
Do you know the 5P's?