If you are truly serious about true, low and slow barbecue then you will need to take good care of your equipment. To get the most out of your smoker investment you need to start caring for it from the first time you set it up. Many smokers need to be seasoned, most will need periodic repair and repainting and all need to be cleaned regularly, and by regularly I mean after every use.
Vertical water smokers, propane smokers, electric smokers and the like typically can be used without a lot of preparation. I do recommend with any smoker that you fire it up without food once to get the hang of using it and to help burn off any residue from manufacturing. Larger smokers usually need to be seasoned. Your instruction manual will tell you what you should do, but the basic procedure is the same no matter what. Start by reading the manual to find out the best method for preparing your smoker before you use it.
Seasoning a metal smoker is like seasoning a cast iron pan. Coat the entire inside surface with oil. You can use practically any kind of cooking oil from Pam to peanut oil or even bacon grease. The kind of oil isn't going to make a big difference so don't waste a lot of money on it. Once you have a good coating of oil you need to heat that oil to a temperature that will allow it to seep into every imperfection in the metal surface of the smoker. This will create the barrier that will repel water and keep your smoker from rusting. Heat the smoker to a temperature around 250 to 275 degrees F. Any temperature higher can damage the paint on your smoker. Many smokers, particularly the cheaper vertical water smokers can shed their paint at temperatures as low as 300 degrees F.
If your smoker doesn't require seasoning I still suggest that you fire it up once before you smoke so that the metal is heated to a temperature above 250 degrees F. This helps eliminate any contamination from the smoker and helps you get the hang of it. Whether or not you are seasoning you need to use smoke. Smoke helps create a protective surface over the smoker that will prevent rust. The smoke residue, like oil, repels water. Allow enough ventilation to keep the fire going and to make sure that you don't get a layer of creosote. Creosote is a nasty substance that builds up in a poorly ventilated smoker. You never want this stuff in your smoker.
Now it is important that you keep your smoker clean and maintain the protective coating. This is done by keeping the ashes and food build up out of the smoker, but by not scrubbing the smoker down to the metal. You may need to clean out the smoker completely from time to time and re-season it but you want to maintain the oily, smoky surface in the smoker to prevent rusting. But I'm serious about keeping the ashes out of the smoker. Don't let the ash sit in there for long periods of time. Ash absorbs water and oil and can cause rusting in your fire box. Grease can also trap water against the metal so any large deposits of grease need to be scraped out. I find that scraping away the grease gently solves most problems.
When you clean your smoker, always keep a close eye out for rust. You should give your smoker a good inspection from time to time to make sure that you don't have any rust forming. Rust needs to be completely removed as soon as you spot it. Scrub it out with a good wire brush and maybe some sandpaper. Clean the area and immediately repaint it with a heat resistant "barbecue" paint. Try to use a good quality paint, it pays off in the long run. Remember, when it comes to painting metal you need to get down to the bare metal before you paint of it won't last.
There really isn't a reason that your smoker can not last for many years, particularly if you invested a good sum of money in it in the first place. Remember that your smoker is your key to great food. Take good care of it, use it regularly and you will not only master great barbecue but your barbecue will just continue to get better and better. One of the biggest secrets of great barbecue is knowing your equipment so your investment in a smoker is more than just financial, it's the time you've spent using it and the knowledge you have gained of the individual differences of your particular smoker.