You know that you have 12 players on the bench and 11 on the field. You want to keep those 11 starters at 99 all the time and it's such an easy way to do it. 1: Buy 2 teams or 23 players. Make sure for your formation you have "2" players at each spot. Example: I have Pepe at CB and I also have Kompany too. And do that for each position 2: Play a game online and lose fitness, your starting 11 will be below 99 now 3: now from step 1 you had 2 players for each position, one sat on the bench and the other started the game. Now just 'swap' those players. Example: I switched Pepe out for Kompany in his place. 4: now play a game with the bench team you just put in. You will notice when the game is done your first starting 11 on the first game will be back at 99 fitness. Why? Because fifa implemented the idea that players on the bench to get fitness back up!!.
Works very well 2 teams is best - Use main team in leagues - then use a cheap sweaty team in the cup - and have your league team bench & reserves - easier than keep swapping whole 11 players in my eyes