First, you shouldn't hold your breath waiting for future periods to trade at $40/bbl.  Right now, the NYMEX futures market for Dec 2012 (furthest out contract) is $57+.  To get that to drop to $40, the front end would likely be more like $30.  That's pretty unlikely. check out nymex.com for info.

If you really want to invest in oil, you can get an account with some stock brokerages to trade on NYMEX("the Merc").  They may want you to prove you have enough trading experience to handle such volatile and risky instruments.  Be advised that the margin requirements are not trivial.

If you want to trade the smallest increment, 1 lot of WTI, you are trading 1000 bbls.  If you go well out the curve, the margin requirment of the NYMEX is $2500-3500 per lot.  So you are putting up about 10% of the value of the oil underlying the contract.  That kind of leverage has pretty sharp risk/reward teeth.  As the trade moves against you, you have to put up more and more cash as margin;  you have to put up more cash as the contract becomes prompt(more volatile) too.  If you call it right you can make big % gains.  A broker may have higher margins than the Merc requires.  I've never tried as an individual so no direct experience.

There is a mini contract on the Merc but it's only half the size of the regular one.  It cash settles so at least you can never be forced to play with "wet" oil although no respectable broker would ever let a neophyte get anywhere near expiry still holding a prompt contract.  Actually, they'd probably force you to close the trade as it would be their ass on the line with the exchange when you couldn't perform.

There are some other things you can do.

1.  Buy oil trusts like BPT.  I once bought this at $4/share.  Now trading at $70 with huge quarterly dividends.  You are buying a piece of the cash flowing from the ANS field.  and no, I wussed out when I'd doubled my bet.  There are others out there but read the prospectus on anything of this sort VERY carefully.  This isn't an investment for the lazy.  You can lose all your money pretty damn quick.

2.  Consider shares in oil production companies ie, oil cos without a lot of refining (like UNOCAL used to be before Chevron gobbled them up).  Also drilling/service companies.  Both are trading pretty high.  I don't have the stomach to invest in these folks at these prices.  Not that many are left anyway as the integrated oils keep buying them up.

 Be careful.  There are many reasons 7 year oil is only $57 with the prompt over $61.  Jerome has many good points, but they are only one side of the argument.  Only 6-8 months ago, people on these blogs were all hot to buy Euros at $1.35 because they "just had to be going to $2.00".  Currently it's more like $1.18 so keep that in mind.

have fun!