A couple of years ago I was at a wine judging where the judges had agreed to evaluate some locally produced artisan spirits. They were unhappy that the glasses we provided were wrong, and I proceeded to watch them use the perfectly shaped chimneyed spirits glasses to delicately sip a variety of spirits and spit them out. Their evaluations were fair but their technique was not. There are some major differences in the way a spirit is tasted from the way wine is.
1. How to smell.
The proper glass for spirits will have a wider bottom that fits nicely into your palm so you can warm the spirit up, promoting volatilization and enhancing the aroma experience. It will have a narrower opening with a bit of a chimney to funnel those aromas toward your nose. The exact shape will vary as cognac snifters are quite different from whiskey sniffers, but the shape pattern is consistent.
Warm the spirit in your hand, then sniff gently, rolling your head back and forth so that you can focus on one nostril at a time. This way, if one is a bit clogged that day you'll get the full experience. Sniff a few times then take a break and clear your nose and repeat the process 3, or 5, or 8 times until you are satisfied that your aren't getting anything new out of it.
Make sure to get your nose as far into the glass as possible. Your sniffs should be short and shallow not big full lung inhalations. It's easy to get a nose full of alcohol and burn your sensory out, decreasing your sensitivity and forcing you to wait for recovery. Remember ethanol is a topical anesthetic so it will make you go numb...in more than one way. Another thing that can help is having a pitcher of water handy to smell. A nice big surface of water to take a big solid inhale of can restore your ability to sense the less obvious substances in your glass and soothe some of the burn. Some people say keeping your mouth slightly open will enhance your ability to detect subtle aromas. I like to allow just a tiny bit of air into my mouth as I sniff as that seems to help move the air flow through my sinuses.
I'll also say that there are some new spirits nosing glasses in completely different shapes. I haven't really had a chance to review them, but will perhaps add to this once I have.
2. No spitting.
Take a small sip to acclimate your mouth to the spirit and let it sit for 15-30 seconds and then swallow. This first sip will give your pallet time to adjust to the ethanol and prepare for the next step. Now take a larger one, but no spitting please. One of the primary points of experience when tasting spirits is what happens after you swallow. The spirit rolls down the back of your throat on your tongue and volatile components travel up into your sinuses as vapor, where the real experience of "tasting" begins. In spirits some of the components easily volatilize, some less, but it is impossible to fully taste a spirit without the swallow. With wine tasting the mouth fulls are so large that some of this occurs even when you spit. Not so with spirits as you'll not be filling your mouth with the spirit...at least I hope not.
Ever heard of the bourbon chew? The amount of spirit you take into your mouth for tasting will be less than a mouth full and the best way to get the full experience is to tip your head back with your eyes closed and chew on the spirit, opening your mouth as you do so. This enhances volatility and allows you to experience all of the flavors there. A good spirit will have you chewing for a while before all of the flavors are gone. Mature spirits have higher molecular weight compounds that take time to evaporate from your pallet. The chewing helps, and in general the longer a spirit has aged the longer it will take to fully experience it, and the longer it will linger on your pallet. This is one of the major markers of maturity.
While those delightful volatile aromas are wafting up through the back of your oral cavity to contact the nerves which register aroma, don't forget to pay attention to the effects the liquid has on your mouth. The surface of your mouth will register the 5 classic tastes (sweet, sour, salt, bitter, umami) plus a variety of chemo-sensations (astringent drying/puckering, burn, pepper, cooling).
4. Drink lots of water if you are tasting multiple spirits.
This is a good idea when drinking anyway but the true reason in terms of tasting has to do with the topical anesthetic properties of the ethanol. A series of tastings will numb your mouth and reduce your ability to taste and water will help to mitigate this.
5. Don't automatically add water or ice.
At 20% alcohol it is much easier to smell some trace compounds in a spirit but almost impossible to taste the character of the spirit as it was bottled. Some producers use this 20% as a way of detecting certain faults by nosing but it is important to note that the distiller put it in the bottle the way he intended it to be experienced, and you should give a shot to tasting the way he did the day he hunched over his barrel and called it ready. I'd say that part of a complete evaluation is to taste and smell a spirit in a variety of contexts but for strictly evaluating one based on the qualities of the spirit as it is, warm and at bottle strength is a good place to start.
6. Everything you eat or drink that day will affect your senses.
Coffee in the morning can really decrease your sensitivity even 2-3 hours later. If you're going to be sitting down for a tasting, make sure everything you eat or drink is very mild before the tasting. This will make sure you're sensitive to the most subtle flavors your mouth and nose can pick up.
These are some tips I use. I'm sure a more experienced taster would add to this list but I've found these to be good rules of thumb.