Using canned spray paint can be daunting to many, but if you know how to spray paint, it will result in a perfectly finished surface that will last for years.
Clean the Surface
Cleaning the surface is of the utmost importance. Use a good quality commercial cleaner to prepare the object, and clean off all of the dust and grease. Once the cleaning agent is completely dry, use an adhesive cloth (tack rag) to remove any more bits of dust prior to moving onto the next step. If you are painting a metal subject, it is wise to use a "non-rusting" or "rust removing" primer for this bit.
Apply a Primer
A primer is basically a thinner version of a flat paint, that, when applied, ends up with microscopic "teeth" that help the paint adhere to the surface, and keeps the finish coat from peeling off over time. Primer comes in spray cans, as well as a brush on liquid, typically in gray or white. Use white if you are going to spray a light colored final coat, gray if you are going with something dark or metallic. You want to mist the spray, not cover the object too much, remember, this is just there to give the final coat a good hold on the surface, not a thick finish.
Agitate with Paint and Solvent
Knock down any rough spots after the primer has dried thoroughly, typically for at least an hour or so. If you touch the item, and leave a fingerprint, it is still too wet. Tack rag the object again, and begin shaking up your spray paint. A minimum of three minutes of constant agitation is normally required to get a good mixture of paint and solvent.
Spray the Paint
Much like you did with the priming coat, begin by misting the item from a distance of over a foot, lightly making passes. It helps a lot if you begin spraying before you are pointing at the object, and keep on doing so as you make each pass, sort of like you are fanning a princess. You should now have a painted item that has a good base color that can be built upon. Most light colors will require at least three layers, darks will only take two or so, and the metallics up to five. Make sure each set is dry before reapplying more paint!
Good spray jobs are built up in layers, allowing for each additional pass to dry, and then be added upon. If you slowly make passes, the color will start to brighten with each new dry coat, and you should have absolutely no "sagging" or "dripping". Don't rush it, just take your time and spray gently.
If you are spraying an item for outdoor use, pick the appropriate exterior paint, or apply a clear coat after the final color has cured (12 hours).
Most spray paints are not advisable to be used indoors without adequate ventilation, the fumes can build up quite quickly, and are very volatile. Likewise, do not spray paint in an enclosed area with any sort of gas fired heater or appliance. The vapors quickly break down, and are easily removed with a fan. It would also be prudent to wear a mask and eye protection, just in case you manage to get the spray button turned around.
Once you let the object dry for up to 24 hours depending on humidity, it should be able to stand up to years of use.