Here are few tips:

 - organise your staff as much evenly as you can,
 - put some items between clothes to not waste extra space when they're next to other items,
 - use the extra space between and next to handles underneath the bottom (if present),
 - 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, so they won't ask you about something they won't see,
 - 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 or security clearance always choose the way with people who looks nice and easy or check how they treat other passengers before moving forward (statistically usually man-woman, woman-man rule works, so if you're man - go to woman and smile nicely),
 - be self-confident, walk fast, passively follow crowd, not distinguish between persons, don't exchange staff eyes (be for them another millionth boring, ordinary passenger),
 - always keep your risky bags behind you and far from the staff who can think about your bag and stop you,
 - if you see bad timing in the front of you, ask friend to distract staff by asking for gate number and bypass staff as fast as you can,
 - 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.