Is it due to the heat? I know that I retain about 1-2kg of fluid on really hot days. If that's the case then there's really no solution except to wait for cooler weather.
You've mentioned reducing salt, and it's important to remember to not just reduce added salt but also the salt that sneaks into your diet through manufactured foods. Tins of diced tomatoes, for instance, vary wildly in the amount of salt added. I was amazed when I started checking sodium content in what I previously thought to be healthy foods.
Make sure you're drinking plenty of fluid. Sometimes the body holds onto water if it thinks it will become dehydrated (hence the fluid gain in hot weather) if you cut down on liquids too much paradoxically it can make you retain more fluid.
To reduce the effect of salt on your body you could increase the amount of potassium you eat. I think adults are supposed to get about 4.7g of potassium each day, but most people don't eat nearly that amount. Foods that are rich in potassium include: meat, apricots (especially dried apricots), avocado, bananas, cantaloupe, honeydew melon, kiwis, milk, oranges and orange juice, potatoes, spinach, tomatoes and winter squash.
Good luck! I hope you get your fluid retention sorted.