I received a job offer from company A today but I am anticipating possibly receiving another job offer from company B later this week.
What should I say to company A? Obviously, I don't want to lose A's offer if B does not make a better offer.
I was in this exact situation recently, and the best thing you can do is tell A that you are waiting on another offer and give them a time frame, i.e. I will let you know at the end of the week.
You can tell A that you would much rather work with them but you want to hear the offer from B just in case there is a significant difference in the money offered. This way they won't think that they are a second choice and potentially drop the offer.
If A has a problem with waiting they will tell you and then you will have to make a decision, otherwise you can wait to hear from B and then choose whichever offer is best for you.
When I did this A asked me to give them a chance to match any offer made by B, easiest negotiation ever.