First, it is very important to properly prepare the stems before putting them in the water. Ensure you cut the stems at an angle to increase the cross-sectional area and allow for better water absorption. Removing any leaves under the water is also worth removing, as they can cause rot and reduce water quality.
Secondly, use clean water and add special nutrient solutions that often come with flowers. If these are unavailable, you can make your solution by adding some sugar, vinegar, or lemon juice to the water. This will help the flowers get the nutrients necessary to prevent bacteria from developing.
Finally, place the bouquet in a cool place, away from direct sunlight and heaters. This will help prevent the flowers from wilting quickly. If you need fresh flowers, I recommend looking at https://myglobalflowers.com/international-flower-delivery/thailand/chiang-mai, where you can find many options with quality assurance and fresh flower delivery in China.