Limit the number of new toys that you buy. It's "normal" to lose interest in new things. I suggest that you put away many of their toys and bring them out again later. Rotate their toys in and out of the "toy box." That way some will appear "new" and They'll once more be interested in them.
It is not a parent's job to provide everything a child wants. You take charge of the toys and decide what you can afford and what is beneficial for your kids. They need to learn boundaries and limits when it comes to getting new toys.
You don't give your kids ages. If they're in grade school or older talk with them about your observations. Suggest that they decide what toy(s) they know they'll stay interested in for longer and say that you will buy certain toys but not others.
Provide other interests for them if they're older. Take them to science and children's museums. Expose them to hobby type toys. If they develop a specific interest their desire for multiple toys may naturally be reduced.
My grandchildren are now 7 and 10. From the time they were little I've bought toys for them from thrift stores. Because they cost so much less than new toys I don't feel bad when they quickly abandon them. I just donate them back to the store. Perhaps you can teach your kids to accept second hand toys in good condition.
I suggest that you also encourage them to donate toys in which they're no longer interested. I do keep some toys for several months because they frequently become interested once again.
Later: Liz is right on. Sharing the fantasy of having the toy makes not getting it easier. As she said, your child knows you've heard him and sympathizes with his wanting it.
After reading about a Christmas and Birthday list, reminded me that I also do that. With my 7yo grandson he immediately started asking to put toys on his birthday list the next week after his birthday. He didn't mind that he didn't get the toy. He did remind me periodically that it should be on his birthday list but didn't beg to buy it now. There are some toys he still begs for but sharing his interest in the toy and saying I'm putting it on your Christmas list now, allows us to leave the toy section. For the last couple of months we've been going to the toy section to just look and for the most part that has helped because of our making a Christmas list.