Two words: PACKING CUBES
They really help you pack a lot of stuff into a small space.
Check out eBags...their brand of luggage is A-mazing...and it all has a lifetime warranty. They have short videos that demonstrate all of their products and it really illustrates how easy it is to pack a lot of stuff in a small space. I *love* my TLS MotherLode Convertible Junior. I bought one for each member of our family and we can EASILY fit enough clothes for a week in each bag...and they fit UNDER THE SEAT on SWA planes so we can call them our "personal item"...meaning we can still take a carry-on bag for the overhead bin. The plastic clip on my daughter's bag broke, rendering it unusable as a backpack, but perfectly fine as a shoulder bag or suitcase. I contacted eBags customer service and they sent me a BRAND NEW bag! I didn't even have to return the "broken" one! They did the same thing when we had a zipper break on one of our eBags lunch bags. They 100% stand behind their products...best customer service I have ever experienced. (On a side note, if you have an AMEX card, you may qualify for a free Shop Runner shipping membership. This would give you FREE 2-day shipping from eBags! A lot of their products ship for free anyways, but not always in 2 days)
I will echo what others have said...don't rely on others' experiences to decide whether or not you want to test the limits. If the airline decides that they want to measure your bag, and you are oversize, you will have to fork over the oversize fee. If you have an oversize bag, expect to pay the fee, or consider yourself lucky if they decide to overlook it.