How to optimise your odds to survive in expert difficulty?

• Play with a team - Bots won't help you here. You need a team of people that stick together. Some players prefer playing with 1 bot on their team to spot/take care of special infected, but personally I believe the downsides of a bot outweigh this upside, as a coordinated team should have little issue hearing, locating and dealing with special infected. Use microphones, talk to each other, coordinate efforts, and you'll find that 4th player a lot more beneficial than an A.I. bot.
• Don't run off, but don't stick together like glue - Don't get so far ahead that your friends can't rescue you, but also don't bunch together in a group, as you'll all get hit by boomer bile, knockback from hunter pounces etc. Also try to pass one-way obstacles at the same time, if one person gets pounced on the other side they're pretty much dead.
• Turn on full subtitles & listen to audio cues - There's certain musical pieces that play when hordes come, hunters in the area, that sort of thing. They also show up as text warnings with Full Subtitles turned on in the settings. Use them as early warning markers, for example, retreat to a small corridor/room with a door when you hear a horde. Keep your eyes peeled for hunters if you hear them etc.
• Shove, shoot, reload. Don't get caught with no ammo in your clip. You can reload whilst shoving, so make sure you do.
• Health packs - You only get one per level, so don't use it unless absolutely necessary. Usually this means waiting until you've been incapped twice, and are on "Black-and white" (there are some exceptions, most notably if your entire team is low health). If someone is on black and white and doesn't have a health pack, heal them up. You need a full team for as long as possible.
• Pills - Unless you are finding an overabundance of them, don't use them unless you're less than 50% health to receive the maximum benefit. Spread them around as your team needs - no point waiting for someone to limp along with 1HP if someone else has pills.
• Learn item spawn points Obviously this comes with experience, but there are certain pre-determined areas where items have a chance of spawning. They differ from level to level, but generically, check:
  • Short dead-ends (roads/alleys blocked off by debris or cars),
  • Kitchens and bathrooms (apartments/houses),
  • Shelving units (offices and garages)
  • Park benches & seats

A lot of it comes down to experience and luck, so don't underestimate that. Even the best players sometimes can't finish expert and that's OK. Just be ready to spend a lot of time dying before finishing a level.

The first rule about expert mode: more than anything else, avoid friendly fire AT ALL COSTS, especially if you're using a shotgun. In expert difficulty, each projectile can take 10hp away, and since a shotgun fires 8 (normal shotgun) to 10 (shotgun auto) buckshots, it can instantly incap one of your teammates.

Sometimes, playing with a bot in your team can help you:

• AI can easily aim at infected that human eyes can barely spot, like a hunter in a tree, or any special infected hidden in Blood Harvest bushes, for instance
• actually, AI easily spots a lot of things that you could miss (simply because you are rushing or because you are caught in a horde)
  • try to listen at their "Look!" verbal orders as much as you can
• AI won't spoil precious molotovs or any grenade stuff, so while playing with AI, use it as often as you can
• AI reviving a fallen teammate won't be interrupted if he gets hit: you'll be better off protecting the AI and allowing them to execute the revive than doing it yourself

Also you can't spam your secondary melee attack in L4D 2 (while it was possible in the first L4D), but you can use the very powerful melee weapons to get rid of free hordes of zombies without spending your precious ammo.

Finally, note that dead survivors will respawn at the next stage with 50hp. So if they are not carrying precious weapons or stuff, any survivor below 50hp and without healthpack can be taken down at the end of the level.