•Don't wash your hair everyday
When I first started dyeing my hair, after I dyed it I would rinse out the dye and immediately washed it but that translated in the color lasting a week or less I had to redye every single week, it was inconvenient to say the least, then I looked up how to go longer without washing your hair and started "training" my hair so it wouldn't get greasy after one wash and stopped washing it everyday.
•Co-wash
Basically co-washing is my best friend, washing your hair with conditioner only helps your hair maintain the color longer, plus it keeps your hair healthier since you're not stripping it from all the essential oils it needs. Be careful though, you need to find the conditioner that's right for you otherwise you'll end up with super greasy hair, there's also products made exactly for co-washing. I shampoo with a sulphate free shampoo every 5-6 co-washes to get rid of the build up.
•Watch out for sulphates
I know a lot of people hate sulphates and my main problem with them is that they are the enemy of semipermanent dye, sulphates make your color wash out like crazy so if you use shampoo or conditioner with sulphates your color is gonna last about 3-4 washes.
•Avoid heat tools as much as you can
Heat makes semipermanent dye fade, especially if you use it everyday and in a high temperature, if you really can't give up heat then try a lower temperature and a thermal protector for colored hair.
•Put dye in your conditioner
If your hair is all the same color, you can put some dye in the conditioner you use, this will help deposit more color everytime you use it.
•Sun is not your friend
Try using a cap if you're gonna be out in the sun for a long time, when I had orange hair it faded to yellow in about 2 weeks because I was constantly in the sun, it acts as another heat tool plus sun is damaging for your hair so if you can, cover your hair.
•Cold water
Try only using cold water in your hair because as I said earlier, heat is not good for your hair. What I do is I clean my hair first in the shower with cold water (or lukewarm if you can't handle the cold) and then keep my head away from the water and warm it up so I can finish showering.
•Dry shampoo
I know this applies for everyone, but dry shampoo really is your best friend, just watch out because some aerosol ones have soaked the color and left me with weird faded patches of faded hair.