Can I do something for being heavier to be lifted? Playing with a friend, one started to try to lift the other, and then the other way back.
We started to talk about "making some force" to be more difficult to lift - one could think about having more weight, but I don't think that's possible at all :)
Is it true that if I tense my muscles - I think that was what we were doing - I become harder to lift? Or is it just that, I don't know, the position of my body makes it harder - but not because of anything related to forces?
What are the physics involved?
 A: The weight is always the same.
You can facilitate or not the action of lifting.
Take the weightlifting sport as an example: the weight is taken from below and the centre of mass (CM) is under the athlete  as much as possible. So, it will be harder to lift if you increase the distance between you and the other in order to move your CM away.
Move down as much as possible too to make bigger the distance between his and your CM and he will have to bend his back.
Move as much as possible your weight to one side in order to increase the effort your opponent will have to do with a single arm. 
As a private joke ;) : Of course, to be more effective apply your knee in his balls and the probabilities turned in your favor.    
A: This is something which I recently found out about, yet I have no idea about the physics behind. If you point your toes down (by pushing your toes and ankles to be perpendicular to the floor while standing), you become a lot more difficult to lift.
I can easily lift one of my friends, but if he points his toes down it becomes a lot harder to do. Sadly, I'm not sure why, but at least it's something!
A: My other response addresses the question of forces.
In this response I approach the issue of perception of exerted force. It happened to me in an experiment when still a boy and it took me many years to understand.
A young man sat down and four others were around him, myself included.
We got up it simultaneously using only two fingers of each hand applied to his arms and legs, and he was very heavy, ie had the feel of this.
Then imposed at the same time our hands on his head. With our arms stretched and alternately kept our hands, eight, over his head so that not touched, for several  seconds.
After that we we got up it again the same way and this time it was much lighter, ie we were very clear that feeling.
Only many years later I managed to understand the mystery of what had happened:  
Before the second lifting action we did a calibration, ie establish a high point of reference, when we made force with the arms to keep them raised and stretched.
Our mental measure of the force exerted was made by comparison with this reference,  that was already quite closer to the force that we exercised.
In conclusion: We are sensitive to differences and not the absolute values.
The fact that our scale is logarithmic, not linear, is not relevant in this case.  
