A sampling of Harvard economist Greg Mankiw's advice for Obama:
[D]uring the campaign, you promised that you would cut taxes for 95 percent of Americans, that you would vastly expand health insurance coverage, and that you would never cut Social Security benefits or raise the retirement age. You will almost surely have to renege on some of these promises. As your economic team will often remind you, even if the laws of arithmetic are ignored during campaigns, they provide a real constraint when making actual policy.
The rest is well-worth a read. It is here.