Hopefully Justice Ginsburg will be fine, and stay on the court as long as she wants, but more likely than not the President will get to fill a few spots on the Supreme Court. Here is how he should approach it:
Depending on who retires or what spot is vacant, there should be a different approach to maintain diversity in numerous ways including ideology. The court should not be moved dramatically to the left. (I assume that it won't move right under any circumstances.)
So if any of the Conservatives leave (Scalia, Thomas, Roberts, Alito) the President should try to appoint a more moderate voice from the legal community. Not necessarily a judge or lawyer but someone who is not of or from the left-wing. In a political sense you would want someone with the politics of a Harold Ford or Sam Nunn.
If any of the Liberals leave (Ginsburg, Stevens, Breyer, Souter), the President should try to appoint a liberal thinker such as Alan Dershowitz or Lawrence Tribe.
If Kennedy retires the President should appoint another moderate, maybe even a political independent, someone whose politics would be similar to a Colin Powell.
I mention these picks in terms of politics because ultimately on the cases that matter, that is what the court has either evolved or devolved into. People basically are making political judgements about what is possible in society, what the Constitution and the Law says, and what is in their best interests.
I don't wish we had a more pristine system, because this way the court is judicial enough to have political credibility, and political enough to make good judicial decisions. We shouldn't have courts that make Constitutional decisions irrespective of the political climate. So that means the court is a political battleground.
While I support the President, I hope Obama doesn't pack the Supreme Court or the lower courts with leftwingers:
The people who think "marriage" is discriminatory if it isn't redefined to include perversities.
The people who think the death penalty is too cruel, but abortion rights are just fine.
The people who think campaign finance reform rules are just great.
The people who think our Constitution applies to terrorists.
The people who think "God" should be removed from the public square
...and so on.
I hope the President picks as good a Judiciary as he had done with his Cabinet.
The Democratic Party is now the Majority Party. We should work hard to build an America we can be proud of years from now. This will involve promoting:
traditional values like marriage, hard work and honesty
discouraging frivilous lawsuits
personal responsibility rather than blaming others for failures
equal opportunity for all Americans to succeed
...and so on
Our court system will be very important in defining what is allowable under the Constitution, and those of us in the Democratic Party who tend to stress what's right with America must ensure the Protest Community doesn't run rampant.
making the word "liberal" safe again!