Just remember, try to leave as little energy behind as possible each level. If you get a lot of top mafia wheelman procs (i.e. free jobs), you should make sure you start doing really inefficient jobs, such as the "Destroy an Enemy Mob Hideout" to use up all your energy before you level. Never leave energy behind.
Certain jobs in the "Soldier" tier will be self-sufficient, such as the "Museum Break-in" job. You should be able to level that one up and even use some extra energy on the not so good jobs and still level. You will need to do the museum job a few extra times to get through Soldier. As an example, you might do the Museum Break-in job a few times at the start of the level, and switch over to the Destroy an Enemy Mob Hideout as soon as your remaining energy and experience points to next level are the same number. This way you can master the jobs that don't give good experience while still managing to level up off of 1 energy bar. Soldier is one of the slowest job tiers. Do not get discouraged if you have to wait around a bit during the first few hours to get through the soldier tier. Once you have mastered it, you will be able to level quickly and painlessly through enforcer and hitman.
Enforcer is a very self-sufficient tier. You should be able to master all the jobs in Enforcer without needing a rest and without doing the "Museum Break-in" job. The trick is to just stretch out your energy as far as you can. Do an efficient job like "Burn Down a Tenement" until your energy and exp to next level are pretty close. Afterwards, do the material gathering jobs (ex: Distill Liquor) or the "Smuggle Across the Border" (which is the least efficient job in the tier. Try not to waste energy by doing jobs like "Run an Illegal Poker Game" too many times in one level. When you master Enforcer, you'll have some extra poker chips and cards, enough for 3 or 4 rounds of "Run an Illegal Poker Game". The poker game job is extremely efficient - save these cards. Sometimes, you may miscount or do the math wrong and end up a few XP short of your level. This poker game job is the fastest way to get you up to speed, assuming you already have the reagents needed.
Hitman is probably the easiest tier to level. All your jobs are pretty efficient, and you should be able to level quickly and easily without much thought. If you have extra energy, use it to collect wiretap devices from the Enforcer tier- you'll need them for the Hitman job "Frame a Rival Capo". If you still have extra energy, which you probably will, make sure you use it on the "Rob an Electronics Store" job in the Enforcer tier. You'll need extra cameras for the "Obtain Compromising Pictures" job, plus you'll need a lot of cell phones and computer set-ups for later jobs. You can never rob the electronics store enough!