Here are few tips:

 - organise your staff as much evenly as you can,
 - use the extra space between and next to handles underneath the bottom,
 - if you're taking extra shoes, use extra space inside them,
 - place small item inside packets of your clothes,
 - use external hand luggage packets to store some narrow items,
 - if you're taking any boxes (like gifts), fold the box and the item will take less space,
 - take as much items you can on you (they're not weighing people)
   - take small heavy items from bag into your trousers/jacket pockets (e.g. cables, chargers),
   - dress your-self using the heaviest clothes you're taking with you (e.g. multiple trousers, sweaters or jackets) as usually clothes are taking the most of space),
 - hand on your-self any wearable items (such as cameras) - they're not extra bags,
 - if items are not wearable, use strings and wear it under your jacket,
 - hold in your hands any trouble/big items (such as umbrella, laptop) - they're not bags as well,
 - take extra backpack on your arms, because:
   - you can put big and heavy staff there as usually they don't check/weight the backpacks,
   - when spreading out your staff between 2 bags, it's better than having one big bag with suspicious/unusual size,
   - most airlines they accept small hand-bags (such as backpack), because you can put it under your seat,
   - backpacks are less visible,
   - try Apple bags (with strings), they're almost invisible in the front,
 - if you're going with friend, ask to do the same as above (fill their pockets with extra staff as well),
 - when you're with friend, go to check-in desk separately looking after each other bags, they won't check/weight something that they don't see,
 - when going to departure gates, security clearance always choose the path with people who looks nice and easy (statistically usually man-woman, woman-man rule works, so if you're man - go to woman and smile nicely),
 - when they ask you to measure your bag, go to the next gate and try again,
 - always remember that at security gates they're not care how big/heavy your bags are, the only risky places are at check-in desk and departure gates.