Once approche would be to reduce the amount of connection with the `--maxconnections` argument.
Or your a tor SOCK5 proxy for outgoing connections.
Similar like the patch i wrote for the seeder:
https://github.com/sipa/bitcoin-seeder/pull/29I believe I'm currently running with maxconnections set to 25.
However I'm not running the above bitcoin-seeder app, so I'm not sure how that PR would do anything for me. (I'm only running bitcoin core and P2Pool)