The pit crew's composition is important, as it defines what is important to the character. The character is a rebel? Then you need a strong leader for him to rebel against. He's fighting for the people? Then don't forget to include a sidekick as a representative of those people. Here are your basic five pit crew members, and what they can do for your hero(es). Oh, and I'm going to pick on Batman, simply because he has arguably one of the largest pit crews of just about any hero (ironic considering his, “I work alone” status).
Love Interest: Romance is good; it provides a reason for the hero to do something (protect the love interest, get something for the love interest (to heal or to prove something, or just to show off), can provide conflict (either romantic issues or because the couple is having problems), and connects the hero to the setting (the love interest should not only be representative of the locals, but can also provide updates on any crisis that you have going on and provide an added poignancy. Bruce has a long list of romantic interests, and they have provided their own fun, ranging from hiding his secret identity (Vicki Vale), conflict of interest (Selina Kyle, aka Catwoman), or even your basic “Her Dad's a Villain?” (Talia).
Superior: Arguably the weakest pit crew member, the superior is whoever the hero reports to and who tells the hero that where he will be going. A great character in the right circumstances, too many weak writers create a total nimrod; in essence, the superior is there mostly to show why the military or corporate sucks, and I refer you to my segment on military and religion on things to avoid (note that I'm not saying that a superior can't be used to show how dark the world is, just keep in mind to keep a fine line). However, if used correctly, the superior allows for visible character growth (in terms of promotions, awards, and raises/bonuses), but also things like fun or more important assignments as well as respect. Mostly useful towards the beginning of the comic, and should be shifted to mentor at some point, but can also be a sidekick or love interest, or even a retainer (weirder things have happened!). Although it could be argued that he has no superior, Commissioner Gordon was effectively one at one point (as the Batman was obligated to uphold his status as deputy), or whoever was in charge of the JLA/JLI (as a member (depending on the situation), Batman is somewhat obligated to put in some time helping out).
Mentor: The hero needed to gain his skills from somewhere, and the mentor is that somewhere. Besides giving the hero a reason to prove himself, the mentor also allows for serious character development (demonstrating that the character is growing from student to master to teaching himself). The mentor also provides an excuse for training scenes, which seem to be popular in comics with a martial arts them. The mentor can also send the hero on quests, either for the group that the character serves, or for personal reasons (and the don't need to be serious; Inu Yasha has a monk send his protege on a quest for a special kind of sake). The mentor can also get away with a special kind of exposition: The field report (a briefing on what is basically happening), which is probably one of the least annoying methods of exposition ever created. Although few of Bruce's mentors have been shown, he did learn those ridiculous skills from somewhere. It is worth noting that Alfred used to count (as he taught Bruce most of social skills), as would Commissioner Gordon (even though it's on a technicality; although Bruce is definitely more skilled, Commissioner Gordon has shown Bruce how to be human on more than one occasion, and Commissioner Gordon's tight relationship with Barbara has helped to keep him somwhat romantic, as it's an example of a normal relationship that has worked).
Sidekick: The reverse of the mentor, the sidekick is a character that the hero is training, or as a surrogate son or daughter. The sidekick is best used to give the strip a comedy relief and to lighten an otherwise dark strip, but can also be used to drop a note of seriousness as needed. Bear in mind that a sidekick usually acts as the hero's conscience, as well as someone to bounce strategy off of, and a set of long-distance hands. The sidekick usually has a diminished set of skills or powers based off the hero, but this doesn't always need to be the case (in fact, if you wanted to give the hero a shift in perspective, make his sidekick someone who is there to learn more of the attitude than actual skills, or to chronicle the hero's journey). And keep in mind that the sidekick doesn't need to have respect for the hero (especially at the beginning); it can be interesting to have a sidekick that actually hates the hero yet is forced somehow to be protected or trained by the hero. This includes any member of the Bat-Family (Robin, Nightwing, Batgirl, Batwoman, Spoiler, etc.); the guy likes taking in strays.
Retainer: This covers a lot of evils, but the essentials are the same: The person has skills that are vital to the hero, that the hero doesn't possess himself (such as computer, healing, or administrative skills), skills or abilities that are useful enough to replicate (such as combat skills or super-powers (a lot of bricks start off this way), or even just a different perspective (a civilian in a military group, an observer who can't keep his observations to himself, or even a naïve or cynical character). Keep in mind that this includes soldiers under the command of the hero, people that the hero has hired, or even people that just follow the hero around (those silly friend and companions-in-arms things). Alfred is an obvious example, but so is Azrael (who was hired to take his place prior to going, well, bats). Also, the Sons of Batman (from the Dark Knight Returns) would definitely count as well.