My two cents (which will be long-winded)-
#1 - Training -
When you think about how sideshow sword-swallowers do their tricks, you have to think that it must have required time, effort and training. The throat can be trained as well as the gag reflex. Truth, be told, I wouldn't want to totally get rid of my gag reflex, but tame it a bit. I use toys to train myself. You can also use fingers at first. Just slowly and gently train, taking care not to intentionally gag. If you do gag, it's time for a break in training. This also helps with the initial pain that comes with deep-throating. When it comes to giving oral, gagging can and will likely happen. Which leads me to...
#2 - Pace yourself and accept that gagging is a natural reflex -
I learned to accept that a light gag is normal and not repulsive. I'm not talking about puking or gagging in a painful or unattractive way, but simply pacing yourself and backing off the penis when you gag can help. If you deepthroat and get a good span of time going and suddenly gag a bit, allow yourself to back off and catch your breath without being put out by the fact you gagged. Try not to think of it too seriously. This leads me to ...
#3 - Breath control -
I have discovered that I can ONLY deepthroat when I am holding my breath. If my lungs are empty, I can't go deep at all. Now, every woman is different, but I use this as a guideline. I allow myself to deep throat as long as I can hold my breath. This keeps me from gagging or choking and allows me to take him deeper with less pain. Start slow and get to know your own method of breathing and movement.
#4 - Numbing and menthol -
I wouldn't rely only on these things, but they can aid. They make it more comfortable and help moisten your mouth. Which leads me to...
#5 - Hydrate -
Dehydration is the biggest thing that I have issues with during oral. It's the same concept as sex. If it's dry, it isn't comfortable for either party. Be sure to keep water nearby and maybe some lube. Sucking mints before can help as well.
#6 - Experiment with positions -
Some are easier than others. I go deeper far easier either kneeling in front of him while he sits or laying back, with my head dangling off the bed. (this was suggested above) Do experiment and see what feels best for BOTH of you.
#7 - Flavored Lubes -
If you can score a great flavored lube that you really like, go for it. This, for me, has NOTHING to do with him not tasting good, but it's an extra boost when my mouth is dry. Not only does it immediately lubricate him, but it causes me to salivate a bit, keeping me going even longer. You could probably use anything body-safe that makes your mouth water, but it may not have immediate slip.
#8 - Massage your throat and jaws -
This might sound weird, but it has worked for me. I have deterioration in my jaw and a lot of tension develops there and in my neck and throat. I have found that if I am struggling with deep oral penetration, I can massage my throat, jaws and neck to relieve tension that is causing me to contract my throat muscles.
#9 - Kegels for your throat? -
This is hard to explain, but also helped me and is much appreciated by the men that I use these techniques on. I basically started doing kegels for my throat. Why? Not because it makes it tigher in general, but because it trains me to control my muscles and use them to my advantage as well as theirs. I practice opening my throat, then clenching it. This works best when used with toys, but can be an excellent tool. It adds a bit of variance to deepthroat and gives you a chance to slow down movement, but still stimulate him in an unexpected way. IF I made sense at all, I probably still sound crazy.
#10 (Finally) - Watch when and what you eat -
Eating shortly before such a thing can cause your reflex to be a little more sensitive. Our bodies are designed to expell foreign objects in situations where it could cause discomfort or be dangerous. This is intensified during eating, as our body recognizes a choking risk when we consume food. Not only does this make your gag reflex stronger right after, but ups the risk of stronger gagging and potential vomiting during oral. It sounds disgusting, but it happens. Some foods may be worse than others as well, so IF you are aware that oral may take place, try not to eat for about an hour beforehand. Let your digestive track in your upper torso settle before trying to attempt deep throat.
I hope this helps. Great suggestions so far.