I remove the shower screen and wipe down the screen and plate behind it with a paper towel each night to get rid of any stale coffee oils.
My 1st and 2nd cups seem to be about equal in quality.

I'd like to update my input on this topic. Previously I said that backflushing before pulling the first shot helps, which is probably true, but until recently I've usually only tasted my shots in milk, but when tasting them as straight espresso (with 4g of organic coconut sugar usually), I did notice that the first shot has more bitters in the crema, where the 2nd shot always seems more mild. For cappuccinos, depending on the coffee, sometimes the one with more bitterness is actually better (it helps the flavour stand up in milk). But for straight espresso it's a bit too much.

UltramaticOrange suggested it's due to palate shock, which I think may be the case.
I have a hard time believing some small amounts of leftover coffee oils from yesterdays shot can affect a shot so much.

The mind does strange things to our perception sometimes.
Similar to how our judgement of brightness/colour can be tricked sometimes
http://en.wikipedia.org/wiki/Checker_shadow_illusion


One way of testing would be to brew two similar shots.
Try the 2nd shot first, and see if it's more bitter. Although the 1st one would be colder and had more exposure to air so it could affect things, another way of testing would be to label one as first and the other as second, and wait for them to get to room temperature to be fair (so they're both not so fresh) and then compare. Maybe do the test 2x. Try the 2nd shot first and see if that one's more bitter now.

This could confirm if the 2nd shot is actually always better, or if it's just the 2nd one we taste that's better and our perception is playing tricks on us.

Let me know if anyone tries.

Ben