There 3 types of takeaways you can use:
1) Limiting the time
2) Limiting the quantity
3) Limiting the offer
The first is done by adding a deadline on the offer. A realistic deadline, and not some script that changes everyday. This is done very well when the product or the price is changing after the deadline, or simply no longer available. Take
http://www.blogmastersecrets.com/ for example.
The second is limiting the number of units (stock) or openings (clients) available. Again, back it up with a realistic reason. Perhaps it's "fire sale" for products (products discounted because of minimal cosmetic damage, for example), or depleting stock of an older edition (and to make way for the new).
Or limiting the number of people for a number of reasons -- such as a service provider who can only take on a certain number of clients because there are so many hours in the day, or because it would dilute the value of the service. Etc, etc, etc. Take
http://www.nicheology.com/signup.html for example.
The third is done through limiting other elements in the offer, such as the guarantee, the bonuses/premiums, the price (not a discount, but perhaps an imminent increase in price, perhaps to cover the extra costs in dealing with more customers), the packaging (bundled with other products or components that won't be available after "X" amount sold, etc), the extras (as in free support, installation or shipping), etc.
I like them all (especially when the reason is logical, justifiable and credible... especially when the product is truly limited, such as the Underachiever course I wrote the copy for, which was strictly limited to 700 packages only, and the site was taken down once they've reaced the limit).
But for convenience and flexibility, I prefer the "fire sale" as well as the third (especially with the bonuses). Because often, bonuses can be limited and changed, without limiting the sales of the base product or service.
This not only creates more believability (because it reduces the perception of "control" over the limitation, which may appear as self-serving or manipulative), but also the bonus may be sold elsewhere for real, and the 3rd party may thus put a limit on the quantity to distribute.
I did this with Stephen Pierce, for example, where he was giving away a software program -- one that complemented his infoproduct -- which was being sold by someone else on another website at a real price. Stephen managed to secure only a certain number of copies from the 3rd party.
In negotiation skills training, they call this the "higher authority" or "third party" gambit, where the limitation is truly outside of the owner's control... making the takeaway truly a takeaway, not some ploy.