I spray the racks first with a strong oven cleaner, leave that to start working through the caked on grease, while I put oven cleaner all over inside oven with a sponge/scourer. While thats working on the grime - put oven cleaner on inside glass. Get a glass scraper and making sure you have a good amount of liquid on the glass scrape off all the dirt, it gives a great result and much quicker than a scourer & elbow grease!.
Also use washing up liquid too to give you a base to use the scraper. Use an old toothbrush, knife with cloth wrapped round to get into small areas (rubber rim round oven).
Then have a little scrape at bottom of oven and wire wool sides and back.
If racks are black then use scraper first to get as much off as poss then use wire wool and jif to get the rest off.
Need - Oven cleaner, wire wool pads, sponge scourer, lots of cloths, w.up liquid oh and rubber gloves cos the oven cleaner is strong and can burn your hands.
Hope this helps