I tend to go for plain and simple stuff so the 'extra' is usually just a couple of gems or a scrap of ribbon.
If I have trouble deciding whether to add or not then I find the best thing is to leave the card and go back to it later - either an hour later or the next day. By then I can see it clearer (otherwise I would sit there for hours trying to decide!).
A good tip I discovered was to cut individual gems from their acetate backing sheet and lay them in position on the card. This way if the card doesn't require them they are not wasted (ie don't lose their sticky) and also they don't get accidentally stuck to the card.
I ended up going back to this card three days running because I couldn't get it right. I had stuck the three birds down but then the bottom half of the card looked very bare. I was going to add some red but it didn't look how I expected....
Anyway I ended up using this 'Best Wishes' stamp from a Personal Impressions set (excellent value and designed by the very talented Paula Pascual, love her blog).
Cardstock is DCWV and bird punch is Stampin' Up.