There are several important things to remember about caring for your dog in warm weather.
1. When the outside temperature is over about 70F, NEVER take your dog with you on any trip on which you may have to leave him/her in the car for any period of time. Just a second can turn into a couple of minutes or more, then you've cooked your dog. Seriously, dogs left in cars can suffer from heatstroke which can cause death or irreversible brain damage. Remember, your dog can't open a window or turn on the air conditioner. All he/she can do is pant.
When you do bring your dog on a trip, bring a water bowl and some water. For a Bullmastiff you will need a comparatively large amount. Don't bring a 12oz water bottle, bring a gallon. I use refilled Tropicana orange juice or punch bottles. They are heavy plastic and have screw-on caps, unlike milk jugs.
2. ALWAYS make sure your dog has a clean, cool supply of water. I don't mean a bowl in the yard that you dump and refill every week. Not some sunlit, algae-tinged bowl by the back door. I mean clean and cool supply of water. A deep, large bucket or bowl in a shaded place is required. And clean the bowl often. Remember, if you wouldn't drink it, why are you making your dog drink it?
3. Make sure your dog has access to shade. Shade moves during the day, so you have to make sure that the shade is where the dog can get into it during all daylight hours.
4. Check on your dog several times during the day. This is a good idea no matter what the weather. If you see that your dog is panting unusually hard or is having difficulty breathing, lower his/her body temperature by a dip in the bath tub or a spray from the hose, then take him/her to the vet.
5. Fleas and ticks are most active in warm months. Be sure to be vigilant with your flea control regimen. Have your dog tested for heart worms and give heart worm preventive medication regularly as directed by your veterinarian. Inspect your dog for ticks, particularly after any trips to parks or rural areas.