Does your time machine go both ways? Then just jump ahead a day and buy a newspaper, come back and buy a lottery ticket. 
You could only do that once. The writers of the “Back to the Future” movies had a better idea, get a sports almanac from the future.
Once can be plenty.
This week one of the local lottery drawings was for 40+ million, and no one hit it. I could live for a loooooong time on 40 million.
yeah, there was a jean claude van dam movie about this sort of thing also
Asimovian is a he, by the way.
As regards the problem of getting spending cash from now that will be acceptable in, say, the 1920s: I think you can buy quite a lot of old coinage (pennies, nickels, dimes, quarters) that are too worn to be of value to present-day collectors, for relatively little over the “face value.”
Let’s say that you can put together $500 in coins dating pre-1928, and have paid $650 modern-day dollars for it; all you have to do on arrival in 1929 is go to a bank (if you can find one that hasn’t crashed!), and deposit your real coin. You now have $500 of “good” 1929 money. Hint: don’t put it into stocks.
Given the relative buying powers of that 1929 $500 to your 2006 $650, you should be able to find an easily-saleable commodity to return to our own time with, and profit. Gold has been mentioned–it was at something like $35/oz at the time, compared with about $700/oz today. You could make even more by buying gold coins from the bank that you know are going to be rare–but then you have to deal with the “is it real or fake?” problem that Captain Carrot postulates. The bullion is simpler–gold is gold, and will always have a market.
Your $500 would buy 14 oz of gold (rounded down): return to 2006, sell it for $9800. Not a fortune, but a pretty whopping return on your $650. Repeat as necessary, or start out with a larger stake.
Except for the fact that the next time you go to the future, you find that your $40 million will only buy a pizza and a six pack of cheap beer.
Just go back and buy some futures? OK, so there may not have been 30 year oil futures, but you wouldn’t really need to go back that far. Why take physical delivery of the gas?
WHy not buy stocks? Not every stock crashed in 1929…and even some that did eventually recovered. Why not buy up stocks in companies you know will be worth something 100 years later, open an account to accept the dividends, and then put them in trust…to yourself. Them come back to the future and reap the rewards.
Better to use your time machine to buy up eBay, Microsoft and CISCO stocks (to mention but a few) when they were dirt cheap, then use the time machine to go when the stock prices peak and sell them off.
-XT
I should clarify, you may decide to buy or sell futures depending on how they were trading back then. But given your perfect foresight you should have no problem being on the correct side of the market.
Oh, there would be plenty of stocks worth buying on the 25th of October, 1929…IBM for one, I would think.
But how would you “leave them to yourself?” If they’re in your name in 1929, and you try to cash them in 2006–you’re obviously not the original owner–you’d have to be pushing 100 years old (or better). Questions could be Asked. My feeling is the less attention that you draw to yourself with this kind of caper, the better. No paper trail, thank you.
Better in my view, to take the simple road and deal in an always-saleable commodity–hence the gold idea.
I mean, if it were just as simple as opening a bank account and leaving the $500 to gather interest for 80 years, then so be it.
Easy as pie. 
Simply look up your grandfather, and figure out all his particulars. Then create a trust, claiming you are some kind of long lost relative who wants to leave all your worldly goods to your grandfather or his line of decent. Then go to another lawyer and leave him a sealed envolop to be opened whenever you were born saying that the you in the past had died and all your worldly goods should go to…you.
All perfectly legal. 
Ok, not air tight but I’m doing this between meetings…I’m sure you could tighten things up so that there would be no awkward questions. A trust of some kind though is probably what you’d want to do.
-XT
Not a problem at all. You don’t take your old coins back with you in the time machine; you bury them in your backyard (or some inconspicuous piece of land you know you’ll have access to in the present). Then, you come back in your time machine, and dig them up. Tell the collectors, honestly, that you found them while digging in your garden. If you like, you can add something about how old great-grand-uncle Ned was always a bit eccentric; he must have stashed them away and forgotten about them. The coins will look like a genuine stash that’s been sitting there for a century, because that’s exactly what they are. And old coin stashes like that are legitimately found often enough that it wouldn’t be particularly suspicious.