Here's an easy way to improve the performance of a cheap tripod. The example above is with an old Optex camcorder tripod, which is woefully under spec'd compared to what you should be using to support a DSLR.
If you slip a rubber band between the camera and the mounting plate before you screw the two together, you can increase the friction between them. An added benefit is that the rubber band acts as sort of a damper. It's best to use as wide and flat a band as possible to maximize the contact area.
This won't turn your tripod into a top-range Manfrotto competitor, but it will make the difference between missing a shot and getting a shot. I've used a prime lens in the example above, but this will work with a normal-zoom as well. What you can't do is support a long zoom lens like the Nikon 70-200mm f/2.8.You can even flip the head 90 degrees to shoot in portrait orientation; the camera will hold in place. At least mine did; your mileage will vary.