1) You can either flood the middle with zones using one of the 2-Deep coverages with 5 hook zones (yellow) accross the middle of the field to tighten the windows to fit the pass into... Cover 2 may also be a god idea depending on how your opponent throws the pass.
2) When you call man coverage, be sure to have someone zoning the middle or two guys zoning the seams (between numbers and hashes). Your man coverage will break down as the receiver gets to the middle and will be able to run away as the receiver gets to the seams. Once the receiver gets outside the numbers, man coverage is again fruitful because the sidelines become an extra defender.
3) Don't blitz. Most slants exploit the area blitzing defenders leave vacant. If you leave defenders in coverage, the QB must wait to fit the ball into the receiver. If his timing is disrupted, you have a better chance of defending the pass.
4) Beware the slant cross. If you use man coverage, slants that cross will rub/pick defenders leaving a guy open on the other side similar to the Mesh concept that works so well. You can stagger your coverage depth so the rub doesn't happen, but doing so requires that you pull at least one defender away from the LOS leaving the inside cut open. Be sure to zone the area inside the cut to buy the time you need to bait the offense into throwing where they believe the rub to occur.
5) Shade coverage inside. It will take several adjustments to shade every receiver but usually simply using bump-n-run to jam the receiver will disrupt the timing of the pass while putting your defender underneath the elbow of the slant.
Hope these help... There are tons of ways to take away the slant, but they leave you susceptible to other routes and route combinations. Beware selling out against it unless you know it's coming (track those tendencies).
Later