> The best gear to wear (shoes, shorts/pants)
Wear studs on your feet: e.g., given that it's on a soccer field, studded rugby boots.
> Have them lean back on the rope and walk back dipping their hips
Imagine the following:
#1 Hold on to a rope
#2 Walk your feet forward until you're leaning backward (holding on to the rope) at an angle of, say, 45 degrees, with your back and legs straight(*)
#3 From position #2, without moving any other part of your body, bend your knees slightly ... when you bend at the knee then your body length becomes (effectively) shorter, and therefore your overall attitude becomes more nearly horizontal
#4 From position #3, unbend (straighten) your kmees ... this is the tug, where you're really pulling on (competing against) the other team
#5 Assuming that you won the tug in #4, the other team has given or is giving ground ... so this is your opportunity to take a step back, and goto/repeat from #3 above
(*) I mean more or less straight, because totally straight isn't physiologically possible ... and don't hurt your back: this exercise could be similar to lifting a really heavy weight; so I'd try to practice the opposite of the four faults listed in http://www.pt.ntu.edu.tw/hmchai/Kines04/KINapplication/StandingPosture.files/SwayBack.png
What else ... how do you prevent hands from slipping on the rope? I don't know ... perhaps chalk dust, or rubber gloves.
> What if we have to face stronger team
The first, obviously, is to get lower: you should stand less vertically than the other team, more nearly horizontally.
The second tactic might be to be quicker when the signal to pull is given (if the rules allow that). I used to play in the front row (as prop) of a scrum in a rugby-like game, where the opposing scrum were usually bigger than us ... to win that, we'd get lower (under the opposing row) and push *upwards* explosively as soon as the signal was given (because the other side can't push forward at all while you have lifted them off their feet ... you can't lift them off their feet in a tug-o'-war but the principles of getting lower and doing it earlier, ramping up more quickly, may still apply).
The third tactic is the timing of the heave-ho: if you do #3 and #4 quickly, i.e. try to straighten your legs while your body is still moving downward from #3, then your body's downward momentum too is converted into pull (having downward momentum to arrest makes your bodies effectively heavier).