The biggest thing for me which helped immensely was to stop thinking that 808s have to be clean sine waves. A lot of people on here say that clean sine waves hit the hardest, and yes that is true, but clean sine waves also take up a shit ton of headroom. Usually when I process my 808s they end up being much quieter db wise but with the same relative volume; this allows me to turn them up even more without ruining my mix.
Another big thing for me, is layering. A good trick I've found, is have your 808 in mono, duplicate it, then high pass the duplicated one to remove the low low end so you can then start messing with the content at 100hz and above without worrying about fucking up the low end. This allows you to use stereo effects but keep your actual sub content in mono. After this I usually compress the 2 layers a tiny bit just to gel them together again. I also always resample my 808s after doing this so I don't have to mess with 2 layers any more and it makes it sound a bit cleaner, because it scales all of the phasing effects you may have created during processing. Also I usually have a very long decay on my 808s so you can still play higher notes without the sample cutting off too early.
As for the actual processing, it is key to do all your distortion/saturation/etc in very small amounts. I usually low pass at the end too to bring the volume down on the higher notes, without having to adjust the velocity in the sampler itself.
Anyway, I Hope my wall of text helps someone haha
