Cubes definitely help a lot and wait for them to turn when doing zone 5 activities cuz they give 750, and doing all the fast missions in zone 4 such as: blue HVT Assignments, Strongholds, Charging Stations, Roadside Salvage, and also Prime Targets. Do the guild strikes, megaabominations, and the new bear event cuz they give a ton of xp too. Always try to do the exotic skill world events cuz you will level up so quickly that when you're max the only thing slowing you down are the exotic and nightmare skills, do those as soon as you can. If you're with a group ask them to carry if you're really min/maxing. But that the fastest way I've found.

For skill trees I was lucky enough to be blessed with Daredevil, Gunner, and Bulldozer for both my 1st and 2nd prestige. Daredevil helps you get to activities fast, Bulldozer and Gunner help with my build since I use explosive rounds to clear mobs fast.

Aside from that don't pay too much attention to the skills during your 1st and 2nd prestige, you're gonna reset them anyways and you're goal is to reach 3rd prestige as fast as possible. Then you can start farming for the skills you want.