Depends on the quality and age of your tent,  
For an older tent you first want to buy some seam sealer and seal every sewn seam on your tent.
Then get some silicone waterproof and paint the whole tent.
Let it air out and dry really well, (fumes hang around )
Cut a plastic foot print for your tent to go underneath when you set up.  You should be able to roll it up with your tent.
or still using your foot print, carry an extra tarp to be able to tarp yourself in, it must be large enough to cover an area bigger than the tent so the water can not be blown in.
Even with a new tent I would make a foot print because it will prolong the life of your tent floor.
 

I hang a tarp over my tent if needed in case of rain and angle it so water drains off, Have not had an issue as yet, not even with the wind 

I always pack a couple of 8×8 tarps , hang them from trees over your tent or just throw it over the tent in a pinch . They are also handy to create shade , and use for ground sheets.

In the case of heavy rains and pad slopes, dig a small trench around the outside of your tent to help prevent puddling at the base (a plastic or tarp footprint is also a must!) Make sure to fill in your little trenches when you leave!

Always make sure that nothing (bags/air mattress) is not hitting the sides

When I buy a new tent I make sure to give it a coat of waterproof spray.Also if my tent doesnt come with a foot print, I make one out of a tarp.If I know it will rain I even place an extra tarp over the tent. When I place the tarp I make sure it doesnt touch the tent it self.I usually use trees around to tie it up.

What ever your camp site is like, pitch your tent on higher ground then your surrounding area. With the heavier down pours, you want the water to run away from your tent, not into it. All of the tarps hung over your tent and trenches will not if you are on low ground in a heavy downpour!

I currently have 7 tents, not one of them has ever leaked.  Every few years I spray the with waterproofing, (the older ones) but that’s about it.  If you have a good tent, it should never leak.  If your tent does leak, you should put a tarp on the inside, set up what some people call bathtub style.  Put it on the floor and round up the endges a foot or so on each side wall.  If the tent leaks, this will keep you and your stuff dry, unless it is leaking from above.  If that is the case, just hang a tarp over it.  If you need more help, feel free to email me. Hope you stay dry!