The most important aspect of bobcat trapping is location. You have to be on location to catch a bobcat. Work on using the senses of bobcats against them. They hunt with their eyes and ears first, so use that to your advantage by making your set very visible to 'cats. Then work on their nose, if you can't use something to work on their ears, like a electronic calling device. If the lure peaks a 'cats curiosity, he will investigate the set every time.
Find out what the bobcats are eating in your area and set up travel routes along thick edges to that food source.
Keep the bobcats curiosity up long enough for it to step where you want it to, by using a few different lures or smells at each set. Place the lures or smells so the bobcat has to cross your trap in any direction from which it may approach the set.
The key to catching large numbers of coyotes is to target as many family groups or "packs" as possible. A firmly bedded trap with a good lure or bait on key locations will catch the majority of coyotes on any given area.
Set intersecting points on field edges, crop changes, two tracks and farm lanes. Coyotes will generally take the path of least resistance, so set up on "travel routes" to and from feeding areas, which will be different areas covering up to many square miles.
In my part of the country there are few Red Foxes, but in general if you set up the same type areas that you find coyotes in, those same set types will catch red fox also, as they inhabit the same terrain types.
Grey fox are a different story. Greys are alot more like Bobcats than "canines". They hunt similar to a bobcat in that they hunt thick cover and will even "stalk" their prey. They live in similar terrain such as thick over grown fields, pine tree thickets, cut overs and swamps. Grey fox are light footed and very curious. Be sure to bed your traps solid when you are in grey fox habitat as they will dig at anything that moves under their feet. Block your set in very heavily, as grey fox will try its best to work a set from the side. Make your guiding out of small unstable objects to keep Greys from walking on your guiding.
Flat sets are a very reliable set that can take any type predator. A flat set has about as many different variations as a dirthole set. Backing on a flat set can vary from a bone to a small tree, from a "cow pie" to clump of grass. Almost anything you can think of to get an animals curiosity and/or natural responses to investigate or urinate on an object will work. Once you are on location and have picked a backing, one of the key steps in making a flat set is bedding. Make sure the trap is bedded solid (as it should be in most cases). Next step is to "blend" in the trap bed pattern to match the surroundings as well as possible. A good gland lure and/or urine placed on or under the backing will surely help you catch more predators.
A walk through is a lot like a "blind set" or "trail set" and is placed where the animals normally travel. I use walk through sets for Bobcats and Canines. Both will work a walk through set as well as a dirthole. You can use a walk through in conjunction with a dirthole or flat set. In most cases when I make a walk through I will put lure on both sides of the walk through. This is to keep the animal at the set a little longer and also to stop the animal in the path where a firmly bedded trap is waiting. On walk throughs I usually try to anchor my trap away from the set to keep the set in good condition. This can be done by extension cables connected either to a tree or to a disposable or rebar staking system.