Dakimakura is a great companion to most people, as such we want them to maintain and keep them in good condition. Here are a few simple things you can do to keep them in great condition. 

1. Wash them as necessary. 
Don't wash them TOO often, or they'll begin to fade. A good rule may be every 4-6 months, unless they become very dirty. Are they a bit smelly? Air it out or use fabric freshener (i.e. febreze) on it first! 

2. Shower before bed. 
Some people shower once a day in the morning, some in the evening, some twice a day. For personal hygiene, it is recommended to take showering once a day in the evening. It keeps the pillow from getting sweaty or smelly, and also reduces the dirt it gets on it. 

3. Keep animals away from it. 
We all love pets but they can also do harm to your beloved dakimakura. If you do have a pet, do keep them away from it as for example cats may want to scratch and damage the covers.  

4. Trim (and file) your nails! 
This is common sense and everyone should do it anyway. Some people don't, or they forget. Well-kept nails prevent scratching and tearing of fabric (and self) in sleep. 

5. Keep it off the floor. 
Your floor may appear to be clean, but it is still a factor in adding dirt to the pillow. Avoid if possible.

1. Strip the inner pillow and gather your supplies 
Be gentle when you remove the dakimakura from the inner pillow, they're sturdy but it will still stress the fabric. If you're washing the dakimakura, you probably want to air the pillow out too. You must also decide if you want to wash the dakimakura inside out.  

As for supplies: 
a. Your personal detergent. Just one load's worth, or the '1' line on a marked cap. Make sure you don't use anything with bleach in it. 
b. A small wash basin. You can use the tub, or the sink, or whatever you like, but I keep a plastic tub nearby for cleaning purposes. 
c. Toothbrush. For small, troublesome spots. General soaking and such will remove most of the stuff, but for dried/caked on substances. 

 
2. Fill the wash basin/sink/whatever with COLD water, dump in the detergent and get it mixed up. 
Cold water is important. It will prevent the colors from running. 

3. Immerse the dakimakura in the cold water, get it nice and soapy! 
At this point, if you have any spots, go over them GENTLY with the toothbrush. A little toothbrush action with the detergent will get out almost anything. The most trouble I've had was blood from scratching myself in my sleep, and even that came out fairly easily. 

4. Let it soak for about 20 minutes. 
Make sure the water is still cold. Walk off, let it sit for 20 minutes.  

5. Remove the water, rinse the dakimakura well and dry as best you can. 
These things absorb water like nothing else does! If you don't wring it out, you'll soak the ground beneath where you hang it to dry. 

6. Hang it up to dry for 8-12 hours. 
You can place them in a hanger or your back porch out on the line. Another alternative is to leave it outdoor for it to dry. It'll still be drippy even if you did wring it out well, so put towels beneath it if drying indoors. 

7. It should be clean now!