Use btcjam if you are a long term investor. Otherwise ponzi schemes like hyip will be nice if you afford to loose bcoz buying bitcoins itself a gamble
NEVER do ponzi schemes, you always lose unless you are at the top of the chain! You could rather invest the bitcoin into somthing useful, even gambling is better you could try doubling small amounts to have a small return in money. Whatever you do watch out for hungry scammers though, they are in alot of places online.