The first thing you can try is to just use your imagination, try pretending that on every drive your opponent could hit anywhere. This is a simple approach, but I find can be challenging to maintain for a lengthy period of time. So if this simple idea doesn't work for you here are some adjustments you can make to your length drills and your game will improve.
1. Your opponent has a boast option. If you get it back with a drive (or cross court length) then you now have the boast option and they have to watch closely each shot because you have this in your pocket.
2. If you are doing straight drives only, try allowing a crosscourt length off a volley from in front of your opponent. Although this will still lead to some issues with moving before your opponent hits the ball, at least you both will be holding your ground on the T and not moving backwards before your opponent hits the ball.
3. Use the actual T, yes right up on the T. Having to move this far up will accomplish two thing. You will not time to move back before your opponent hits the ball and you will have to move efficiently into and out of the back corner. Although this may not solve the issue of watching, it should improve your movement.
4. One player can hit straight or crosscourt length, the other just straight length. This will be challenging for the player that can only hit straight and will force them to watch, but it can also let the player hang back on the T. So if you try this drill and you are the one hitting just straight drives, try and fight for the T position and don't give it up so easily!
5. Length game plus 1 short shot per rally each. This will make both player watch closely and if they are hanging back on the T then take advantage of this by attacking short.
6. Length game plus you can go short on the volley. This is a popular one and a good one. No hanging back or getting lazy on this one.
7. Length game and you get points for every volley you hit. Although the front of the court cannot be used here you are in a race to get to 21 volleys. This makes both players fight for the T and because it is competitive it seems to make people watch closer and anticipate.
8. Last one is toughest. One player can hit anything the other can only hit straight drives. Winner of the previous rally can only hit straight drives in the following rally. 8b. Can also do the same but one player can only hit straight or crosscourt length and their opponent can hit anything.
Try some of these drills/condition games for a few weeks. Have patience when trying these because if you have been doing just rotating drives or length games it will take a while to adjust and break out of your bad habits. Remember doing these drills for a few weeks or months will enable you to watch and anticipate better, move more efficiently, cover the court better, will allow you to volley more, and of course improve your length!