Although it's not as great of an idea these days given where rates are (but could still be a good deal depending on your estate size relative to the exemption), one way back in the day (with higher interest rates and a much smaller estate tax exemption) to really reduce your taxable estate somewhat 'quickly' was to buy Savings Bonds. You could purchase up to the annual gift limit each year and name you as the primary owner, and an heir (child, sibling, parent, anyone) as a co-owner. If you have just a few heirs (and a spouse), it can add up pretty quickly for the $/year you can technically shift out of your taxable estate, yet still retain FULL control over.
When you pass on, the heir immediately owns the bond, and simply cashes it in whenever (if they are listed as a beneficiary, all they do is show your death certificate when they cash it in). In the meantime, you as original owner still own full control over the money and can cash it in whenever you need or want to. Otherwise, it can pass immediately to your heir and stay outside of your estate. The only 'bad' part is that if you don't declare your accumulated interest each year, then whomever cashes it in will have a big slug of interest to declare in income the year it's cashed in.
The only 'downfall' of this plan is if one of the heirs passes on at the same time or shortly after the original owner passes on. It's not a huge ordeal - would just require probating the savings bond - but if you have many heirs to divide up your estate to, it can quickly whittle down the tax bill!