This might be a departure from what I normally write about, but I feel that learning how to take naps is very important when it comes to physical and mental recovery from training. In addition, I feel it is beneficial towards general health.

As always, bear in mind I do not consider myself an expert on the subject by any means. These are simply conclusions drawn from my own experiences and techniques I have found successful.

-The "Goal"
Let's be clear about this, falling asleep isn't necessarily the goal of a good nap. I try to think about being able to properly relax my mind and body for a short amount of time in order to replenish my mental capacity and energy. Sleeping is one way to achieve this, but far from the only way.
In fact, I often have successful naps where I never fall out of my own consciousness. Likewise, naps where I have fallen asleep have sometimes led me to be even more tired.
I think in being successful it is important to have a clear idea what you are trying to achieve. Just to sleep is a very over simplified generalization of a nap.

-The Conditions
When I sleep at night I am very picky about things like light, noise, temperature, humidity, etc. However, during a nap I actually prefer different conditions to when I sleep. This helps me to awaken easily and not get too attached to the sleep state.
Similar for me with the preferred napping position. At night I usually sleep on my side and have trouble sleeping on my back. When I nap, on the back is the preferred position. Usually in a variation of the "Savasana" pose from yoga is my favorite.
Same goes with the surface. At home I already sleep on a Japanese futon, which is a fancy way of saying a thin minimalist floor mattress. Napping I have no problem being on a hard floor and most of the time I actually prefer it to a bed, couch, or other similar options.
I think it is very important to be able to separate the concepts of napping and sleeping so your body and subconscious mind know there is a clear difference.

-Physical Relaxation
This is an important concept, as throughout the day we build tension in our body based on different stresses we encounter. Consciously trying to release that tension can be a big step in helping to relax.
This is what I personally think about to help me relax:
Every breath out, I imagine my body melting onto the floor(this visualization is also why I prefer a hard floor and napping on the back). Every part of my body gets heavier and continues to sink deeper until it almost molds itself with the floor. After a few breaths, most body parts will be as far as they can go.
However, the body parts where I hold the most tension will resist melting onto the floor. This allows me to either focus on them more or take a note to save them later for more invasive release techniques.
Common areas I hold tension might be: shoulders, hips, back, biceps, etc.
Also the idea of the relaxation cue goes beyond just napping and can be applied to daily life.

-Mental Relaxation
This one is a bit more complex and will take some individual tinkering to get right. I generally have an overactive mind and my goal is to get it to "shut off". However, it's very important to do this without trying to force anything.
Sometimes when I go for a nap, I am dominated by many thoughts. Rather than forcing relaxation, I let these thoughts go without resistance. When they have run their course, it becomes quite easy to relax and turn off.
Sometimes those thoughts never run their full course through the duration of the nap, but by letting it happen I have still in a sense cleared up some mental space.
Some people may find success by focusing on something like their breathing, but personally I prefer not to focus on anything in particular and let my mind go where it wants to.

-Duration
I usually find anywhere from 20-30 minutes the most effective for a typical workday split. Longer may risk hitting a deeper sleep state, which can lead to grogginess in waking. The only time I nap longer is if I am notably sleep deprived from the previous night.
If I am short on time, less than 20 minutes is also fine but generally not as ideal.

-Tools of the trade
I like to use a sleep mask/blindfold to help shut out the visual stimulus. Some people may like to use earplugs as well, but they don't work too well for me personally.
I also use my phone as a timer just in case. If my nap was performed properly, I usually automatically wake within 25 minutes or so. However, if I have a schedule to keep then sleeping longer is not a risk I am willing to take.
An eye pillow seems like it could be useful but it's something I haven't played with much.

-The "Salvador Dali" nap
I am not completely sure the origins of this technique, but my first experiences with it were from reading about the surrealist painter.
You know when you are driving the car for a long time and start nodding off? The idea is to take advantage of this reflex to create a controlled nap for a one-second duration. This is more relaxing than it seems and is good if you are short on time. It is also useful if you get tired while in the car or sitting at a desk(please make sure the car is safely parked before trying this).
Here are the instructions:
Hold an object gently in your hand while sitting(car keys work well for this). Now simply let yourself drift off like you wanted to in the first place. The moment you fall asleep, your hand will let go of the object which will in turn wake you. The one-second spent in a sleep state is actually more refreshing than it may seem.

-Final thoughts
Remember, napping is a skill. That mean that you might fail a number of times before you start to succeed regularly. Do not worry if certain techniques do not seem to be working; keep modifying until you get something that works for you.
There are no rules, so make your nap what you need it to be for you to help recover and relax.
Also from a higher level training standpoint, I find naps to be invaluable for anyone doing two-a-day training sessions or multiple sessions/rehearsals in one day. This can help greatly with skill acquisition, patterning, and memory with things you are trying to learn/achieve.

As always, I am not quoting any science to back up my claims. This means to take with a grain of salt what you need to from this article. This is a summation with some of my own experiences with the idea that others my benefit from the concepts as well.