I have always loved this photo... it serves as a reminder that everyday most people view you from the rear. To that end, we should all be as considerate of the impression we make from the back as well as the front! As a New Yorker, I could go on for hours about the things I see... not just panty lines, but thongs - with tags hanging out no less!! Frightening. As a Vintage Clothing Stylist, I am often asked how women achieved such smooth clothing lines in the past... the answer is simple: By wearing appropriate undergarments.
And most importantly, Slips.
Slips still exist, but seem to have been forgotten.
On Project Runway, a beautiful dress by Dmitry was marred by a bikini panty line...
If only she had worn a slip under that dress!!
(Bikini panties should not be worn with hourglass dresses - ever!)
Although quality vintage clothing is usually lined, they often need an undergarment to look their best. Similarly, contemporary garments are unlined and made with the lightest of fabrics to save costs, as a result they often cling in the wrong places. Wearing Spanx is one way of dealing with this, but I personally don't enjoy feeling like a sausage. Besides which you have to be a contortionist to get them off -- and you certainly don't look sexy doing so! It is possible to find slips and control hose that will give you a smooth line, enhance your figure and might even your sex life.
Lace top half slips by Vanity Fair are a great option and can be worn on your waist or hip without creating lines. They come in different colors & lengths and usually sell at JC Penny & Target for under $25.
Vanity Fare 18" lace top slips... 
Pettipants are a
great option for full skirts or belted dresses. Such as http://www.herroom.com/farr-west-447-spanish-lace-pettipant.shtml
Ginger Rogers in Tap Pants/Pettipants. 
Full length body hugging slips are a better choice for hour glass dresses, just be careful to check the length of your dresses.
(Who could for get Liz Taylor in hers!)
Most of us tend to favor one length over another. 40" was standard dress length for many years, but in recently mass market hemlines have crept up an inch or 3 to save money on fabric, so they can be anywhere 36-39" I prefer slips with built in bras (like the one Liz is wearing,) but those are harder to find and you will need to try it on before buying (department & specialty stores are better for this.)
Another great solution that will give you smooth lines and sex appeal are Lace topped pantyhose,
such as Hanes French Cut Lace Pantyhose... 
Hue also carries a version of these, but they are ultra sheer and much thinner than the Hanes version. The Hanes hose are also great if you are long legged. You can even get them discounted at www.onehanesplace.com
Lastly, there is a solution for unlined pants... Pant liners. Yes, they are effectively a slip for your slacks and they work - no more cling!
(JC Penny carries them on-line as does the Vermont Country Store.)
Hopefully these tips will help you look your best, from the rear and the front!