is there any way to increase the torque on a linear slide 
I assume you mean a rack-and-pinion setup, you can always add a gear ratio between the motor and the pinion gear to add torque. It will be slower, however.
ummm… I only half follow what your saying
So I’m assuming you are wanting to get more force from the linear slide which is being powered. Is that right?
Basically, to do that, you either need to gear/chain it up for more torque or add more motors.
When you gear/chain something up for a ratio with more torque, you sacrifice speed. Similarly, when you change the ratio for more speed, you lose torque.
To get more force from the linear slides by switching the ratio for more torque, you need a smaller gear on the shaft being powered by the motor, and a larger gear on the output (trying to make this simple for anyone else who may find this post useful).
If this doesn’t seem to help, maybe post some pictures and that might help us to describe a solution 
~George
ok that make a lot more sense.
I relized that mistake after I woke up this morning.
With a rack and pinion you will either need to use more than one motor or use compound gears to increase torque. Is this for a hanging mechanism or scissor lift? If it’s a scissor lift you might be better with a reverse orbital gear set-up (that’s what I call it).
We used four 393 motors, and a lot of rubber bands, for our scissor lift. It can lift three buckyballs or one large ball about 30" off the ground. The top of the lift can almost touch the high bar. You can see the initial lift in action in one of the REX Tournament matches. It needs to be refined a bit for a smoother performance though.
Did you use linear slides with a rack and pinion for your lift? The robot did an impressive job at the REX Toss Up. We used and 84 tooth gear powered by 4 393 High speed motors with a compound gearing that gives us 21:1. We did not need to use rubber bands and get 42" all the way up and can hang with the wheels at around 26"
If you used a rack and pinion your scissor is by far one of the better performing ones I have seen in a while using that.
The lift uses linear slides but not rack and pinion. We use a high strength gear to cause a relative turn between the two lower links on either side of the lift. The gear is torqued by two motors, using a 5:1 gear ratio. So, if T is the torque provided by a single motor, we can generate a total torque of 20T for raising the entire lift. But we still find it necessary to use rubber bands, especially for lifting the large ball.
Deleted one off-topic post.