There are many ways to ask, for example:
Could I please get another X?
May I please have one more X?
Would you please bring me some more X?
The "please" (if you choose to use it) can go various places in the sentence:
(Please) would you (please) get me another X (, please)?
There are many variations of could/can/may/might/would, and get/have/bring, as well as other expressions.
I'd like some more butter and jam, please.
Please may I have some more butter and jam?
You can also add "excuse me" before, and/or "thank you" after, for extra politeness:
Excuse me, but might I ask you for more butter and jam? Thanks.
Excuse me, but could I trouble you for some more butter and jam? Thank you.
If you're unhappy with the service, and you don't feel like being as polite, you could instead use the imperative:
Bring me some more butter and jam, please.
Get me another plate of butter and more jam? Thanks.
This is not a comprehensive list. There are many variations on all of the above examples.