We hire help, persuade NPC equals to help, or do without depending on the campaign circumstances; I chose "NPC equals" because it has occurred most often.
Usually, when we're putting together a party I choose last because I take pride in being able to play any race, any class, any time, no fussing, and if there's an obvious hole I can fill it. But the most recent time we made characters, I got a wildhair to play a monk because I'd never done it. The other PCs are two wizards, a cleric, and a druid. We have recruited former antogonists (my monk has a surprising capacity to convert people) to our cause and also made deals with people to join our party in the old-fashioned tavern-meet manner; plus the DM drops NPCs on us all the time. In the game I'm running, the player with the wizard departed and the party never felt any in-game pressure to replace him. At the moment they're going through Red Hand of Doom and have made a sufficiently good impression on Elsir Vale that they aren't having any trouble recruiting NPCs when they need them, but they play smart enough that getting along on bardic and divine spells plus magic items has served them well.
We've also historically played games in which everyone ran multiple characters, character with followers, or whatever. This tends to cause problems for the DM in the long run, though, as it boosts the power level of the party and the more powerful the party, the harder it is to run a challenging and balanced game. As long as the characters are responding realistically to the situation in the game, there's no need for anyone to die of party holes.