I have a few tricks:
1) Only pack multi-purpose items. Shoes are my hardest thing: What woman can pack just two pairs of shoes for three weddings, five countries, sight-seeing, beaches, and hiking? Not me. But I can do it with four pairs: One pair of silver wedges to go with everything very dressy (weddings), one pair of athletic sandals for sight-seeing, one pair of sneakers for hiking and biking and those days when I just need support, and one pair of silver flip-flops for the beach or casual days/nights. Despite my large feet, all these shoes pack almost-flat. I could have done without the flip-flops, but they take up such little room, I put them in.
Then I make sure all my clothes can mix and match together. Some special pieces, like a bridesmaid dress, won't be remixed, but I can wear it at different events since the weddings are with mostly different people; I can wear it for a semi-formal party, for example.
2). Yes, roll. I do roll things up in my suitcase. It makes them smaller, less wrinkle-prone, and easier to squeeze in so I can see my options without unpacking the whole darn suitcase. (Although the shoes are in a layer underneath against the ribs of the suitcase, so I do need to poke around a bit.)
See that? Formal dresses roll, too!
3). Only pack pieces that are easy to wash. Everything that goes in the suitcase (except the bridesmaid dress) can be washed at a laundromat, and most of it in the sink. Maybe TMI, but I have a few pairs of amazing underwear from Ex Officio means that I only have to pack 1 pair for every 2 or 3 days I am away. (They wash out in the sink with travel packs of detergent or soap or shampoo, if you're desperate, and they air dry within an hour in most climates.)
4). Choose outerwear that can be dressed up or down. I have an amazing little travel raincoat with trenchcoat styling that packs up tiny, but I can't find a comparable one anywhere online. However, something like this Missoula Jacket from Athleta.com or this packable raincoat from TravelSmith would fit the bill.
5). Bring a tiny travel umbrella. Just in case! It fits in a small space but will keep you dry. It also doubles as a sunshade.
6). Carry one purse that will work for everything. I have a chicobag that fits in my daily purse so I can have bonus carrying capacity if I need it (book for the beach, picnic goodies, etc), but I found a fun yet utilitarian purse that I won't feel ridiculous taking to a club, a wedding, or a museum. It fits my sunglasses, my hat, my wallet, my phone, my camera, sunscreen, and a notebook or small guidebook. (For the more formal weddings, I probably will just have my suit-wearing husband carry my necessities.) My one-and-only purse had to be cross-body, with a zip top, an interior zip pocket for my wallet, and enought organizing pockets inside. Maybe my fashion sense is just wacky enough, but just bringing one purse sure simplifies!
7). Find a suitcase that works for you. I love my Samsonite carryon that I got years ago. One side has mesh zip closure where you're supposed to put the garment bag, but I love lining up my fancier things in there in rolled layers instead and zipping it up until I need them.
For a four-week trip, I packed: 3 casual cotton sundresses, 4 tops (short sleeve or sleeveless), 1 lightweight tunic/ coverup, 1 maxidress, 3 tank tops, 4 pairs of shoes, 3 formal dresses (for 3 weddings, 3 rehearsal dinners, and a wedding party), two flowy scarves that can be used as wraps or sarongs as well as a pop of color to tie an outfit together, 3 bras, 5 pairs of underwear, 1 nightshirt (which can work as a dress or beach coverup in a pinch), 1 pair of workout shorts, 4 pairs of compression socks (medical necessity), 1 cardigan, 1 teeny tiny rain trench, 1 pair of washable linen pants, 1 khaki short skirt, and 1 rollable big sunhat.
But don't ask me how many books we packed... I may be efficient with clothing, but I do always have several books as well as an ereader!
What are your tips for packing?