The bait unit is usually a unit that can be a threat from pretty far off so ignoring it will be as much as a problem as will be falling for the bait. Good examples of excellent bait units are Valkyries/Vendettas, Vindicators, Lootas, Hammerheads, and even Assault Bikes/Land Speeders to a lesser extent. Drop Pods can also fulfill a similar role but the payload is usually not juicy enough unless you have troops inside and they contest an objective.
Making good use of reserves and scout moves is another way that your opponent will try to get you to fall for the bait. By holding a key unit or two in reserve your opponent can bring on a juicy bait unit away from their main force and try and get you to divert your attention. This same principle can be applied against you at the start of the game by using scout moves on fast vehicles.
Now that we know a few of the tricks how do we keep from falling for it? One method is to simply ignore the unit and focus on your opponents main forces. This really comes down to figuring out what is the biggest threat and what is going to help you win the game. A single Valkyrie that shoots off from the main force is a threat, but aren't the ordnance tanks, Chimeras full of troops, and who knows what else more of a threat? You probably won't be able to catch the Valkyrie anyway and it is so fast it will get to where it wants later on with not problem.
A lone Vindicator can also be safely ignored if you can move far enough away from it. Vindicators only have an effective threat range of 30". That sounds like a lot but if you are careful with your movement you can minimize the effects of the shot. Taking long range pot shots at it is a pretty bad idea as it's front armor of 13 will be pretty resilient to most guns.
What about those threats that you can't ignore without taking a ton of damage? A big unit of Lootas off to the side is something that you will definitely want to deal with - as is a pair of Hammerhead tanks or two to three artillery platforms. Any long range/high str/low AP detachment can really mess up your plans unless you deal with it fast.
One good way to deal with something like this is by holding a unit or two of your own forces in reserves. A good example is a Drop Pod with a Dreadnought in it. This guy will probably show up on turn three or four and by that time your opponent will probably already have laid their trap. Drop this guy in their middle and unload with a Heavy Flamer and Melta gun and you will get their attention. Most bait units will have a pretty hard time dealing with something like this close up and even if they do it has given your main forces a turn of reprise for a (hopefully) lesser points investment.
Most armies have something similar to this that they can use - Deep Striking Battlesuits, Veterans in a Valkyrie/Vendetta, cheap Firedragons in a Wave Serpent, outflanking Genestealers or even boys in a Trukk. The main point is to make sure you spend less points on taking out the bait than your opponent has invested.
Another good anti-bait tactic is to use something fast/long range to threaten your opponents bait unit. Most skimmers will fall into this category. A 70 point Land Speeder with a Heavy Flamer and Multi Melta is pretty intimidating when it is facing down a single unit or single tank. Your opponent at the least will have to divert fire to take it out.
The last tactic that I will go over is the reverse bait - you make your opponent think you fell for his trap and then turn it back around on him. A Land Raider works perfect for this tactic. You can move the Land Raider off to one side of your army and still take shots with it using power of the machine spirit. If you expect your opponent to bait you can position the Land Raider on the flank of your force. This will let you move 6 and fire a few extra shots at the target. In the next turn you can also rush forward 12" towards the main forces, maybe take a shot behind you as you go, and unload your payload - they will have an effective charge radius of 20" (12" vehicle move, 2" from the hull, 6" charge) this can be a deceptive trick - your opponent thinks you are falling for the bait but then you double back at them with a huge charge.