Find out what kind of vents you are dealing with. They can be ridge vents, which run along the top ridge of a roof to vent the attic and are usually made of metal, off-ridge vents, which look like metal shingles but are raised at the bottom to let air out of the attic, or Goose-neck vents and turbines, the former looking like a goose jutting up from the roof with a hole under the mouth and the latter looking like a cylinder jutting out of the roof with a metal dome on top (similar to a stove-top popcorn popper).
If you have ridge vents, make sure they are firmly anchored. If you pull up on the end of one with around 50 lbs. of force it should not budge. Make sure all the screws are tightly screwed in; you can also add extra screws for added anchorage. If it is nailed in, you can go into your attic and clinch the nails (bend them over) for added support. However, the nails may be too short and stubby to do this, in which case, adding extra screws for stability is your best bet.
Treat off-ridge vents similar to ridge vents. Make sure they are firmly anchored, clinch the nails if you can and add extra screws where appropriate. The key difference with this type of vent is that sometimes (depending on the wind direction) water can be blown up the roof and into the attic. To prevent this, try and block up the opening with Styrofoam or any foam swimming pool float to keep as much water out as possible.
Goose-neck vents are a type of off-ridge vent, and all of the steps applying to the latter should be applied to the former. Additionally, stuffing foam into these vents is essential to prevent rainwater from entering.
Remove turbine vents, as they are not intended to withstand strong winds. Some you can simply remove, and others are screwed onto the ducts, so unscrew them. You can then make a plywood cover for the duct. Using a jigsaw, cut a plywood circle slightly larger than the duct hole. Then screw the plywood firmly into place, making sure the seal is waterproof. For additional waterproofing, you can cut a rubber circle the size of the plywood circle and screw it on between the plywood and the roof.