Keeping two identical or near identical teams is the only way, this is how I do so well!, it does not happen overnight you need to build over a couple of seasons, even if it means buying 4* or old guys just to get you up and running!, play your best team for the league and semi and cup finals otherwise play your second string!, if you get knocked out of the cup/champions league so be it!, in my case I only aim to get to the CL group stages so my second 11 get game time, anything past that is a bonus!.
If you want the cup or cl then prioritize your best 11 for them!, I have tried using my best 11 for every match by using booster packs after every game but it gets way to expensive buying more packs and you dont always win the games, your league starts to falter and you can end up struggling!, take my lead and try to get a full 22 man team, dont worry if its a rag tag miss fit bunch as long as it is a TEAM!, you dont have to make it identical to you best but it helps better if you can as these players double up as back up for injuries for your best team!, try to get multi positional players as this helps immensely!. Try to buy youngsters if possible, then once you have your 22 man team you can gradualy upgrade your players little by little untill eventually you have a great first team and a good second string.