How to stop hair dye from fading...

I've never found those shampoos for coloured hair to be any use, except the Paul Mitchell one, though that is expensive.
Putting some of your dye into your bottle of conditioner helps as it gives a colour boost everytime you use it. Though I guess if you've used a box you won't have any dye left. Might be worth remembering for next time though.

Thank you! That's a great idea.

I recently dyed my hair a darker reddish/brown color, and it already started to fade after about 2 weeks. My aunt is a hairdresser, and she had some pure red hair dye ( and when I say red, I mean RED.) that she gave to me, and I just mixed it with some shampoo and conditioner, and every few days I use it to boost my color. The only downside is that since it is so red, it looks like there has been a murder in my shower every time I use it.

wear a hat in the sun, that helps

Hm there are some good ideas here. The only way I stopped my red hair colour fading (when it used to be dyed red) was to get cheap(ish) bottles of brightly coloured dye, like Crazy Colour or Manic Panic and use it to top up a base colour...

Here are the things I've learned to maintain color it a long time.

Conditioner does not strip color but almost all shampoos will, especially semi perm/temporary dye, and "color saving" shampoos often contain sulfates. Sulfates will strip color. this is highly unavoidable so let's move on to the best way to combat this.

Putting color in your conditioner is a great idea, but remember to still treat it like conditioner and mainly apply it to your ends.

Never allow your hair to be in warm water or direct sunlight. Totally awesome, means you can invest in cute retro headscarves!

Wear a shower cap when you don't need to wash your hair but still have to shower.
When washing your hair, keep the water cold the whole time.

Every time you get out of the shower after applying your color conditioner, the hairs cuticle will be open and still able to lose this added color. Blow dry and if possible straighten your hair(never straighten it while wet though, blow dry first) ASAP, this will heat seal the cuticle and allow it to hold on to the color better.