You can donate to the Bitcoin economy by buying bitcoins and selling them for less than you purchased them. This will increase confidence in the bitcoin economy by decreasing the buy/sell spread. The closer the buy and sell prices are to the mid price, the more willing people are to use a currency since they will feel like they're getting a good deal when they buy or sell bitcoins. The increased confidence will attract more merchants, consumers and developers. To prevent the person who sold you the bitcoins from buying them right back, you can sell them using a different currency.
It might be counter intuitive, but you can actually decrease the spread and profit at the same time.
It the spread is HIGH BID: .05 and LOW ASK: .06 and you place a BID of .052 and an ASK of .058 then you decrease the spread and profit by $.006 per coin. The risk is that only one of your offers is taken and the price moves away from your other one. Possibly leaving you either with BTC of $$ that you don't want, unless you are somehow indifferent.
If you want to give money away to increase the value of BTC then just pick a floor at which to support BTC. Say coins can be sold for .061 right now, choose .07 as a floor and offer that until you are out of cash to burn. If you sell them back in you'll be undoing the manipulation that you did. I don't recommend this btw.
The best thing you could do, imo, is provide whatever good or service you can in return for BTC.
