I know there will be purists who think that the shiny/worn-in look should be kept for originality, but for me its a matter of opinion. Check it out and decide for yourself.
My seats were worn but with no rips, mainly just worn down to the hide around the bolsters. The seats all over were looking tired and shiny with various marks and stains:


For this job I used Gliptone products. First I cleaned the seats and headrests all over using their leather cleaner, then applied the leather conditioner and left to dry. I got myself 1 litre of their "BMW Black" leather dye, which was a good match for my E30's black leather.
For the spraying I used a mains powered compressor and spray gun set - mine were both bought from Lidl but you can rent them cheaply enough, or use the 'hobby gun' that Gliptone do. For best results use a proper compressor and gun like this one, I set mine to 1.5 bar which was about the lowest pressure I could get the gun to work at.


Having washed, conditioned and thoroughly dried out the seats I stripped the seats as much as I had time for and masked up the remaining trim. Then wiped them down all over with methylated spirits (meths). Then it was time to spray the dye on:

This was by far the hardest part of the job. You need to spray several light even coats, leaving to dry between each one. The seats look awful when you are spraying them and its really hard to work out where you have & haven't painted, this is where the several coats help you out. The first time spraying I completely messed it up, put too much dye on and it ran all over the place. Fortunately you can just wipe it all off with a rag and start again - much easier than spraying bodywork!
After all that, left to dry overnight in a dust free environment (my garage had to do). Here are the results - crisp, clean and new looking leather:
Driver's seat:


Headrests looking like new:

Passenger seat:

Close up of lower part:

Couple of the finished item:


Whole job for front and rear seats took me around a day and a half and approx £75 in consumables (leather cleaner, conditioner, dye and meths)