if it was leaning, then it probably has to do with where the light source is located, and how it's orientated....the plant will lean toward the light....are you using artificial lights or sunlight? if it's the sun, then you probably want to get a small flourescent light for night time to train it to stand back up...locate the light where it's opposite of the direction that the plant is bending, so that it bends back the opposite direction...you always want to have artificial lights directly over the very top of the plant, so it's looking "straight up" at it... the sun can be trickier since it moves, but you should have too much of a problem with it...

and if it's not the way your light is positioned and orientated, then listen to the others because their ideas sound good too...

there shouldn't EVER be any reason for you to prop up a plant...if there is, then it's either leaning toward a light, or it's not strong enough...and if it's not strong enough, then back off of the amount of water you give it (they only need water once a week when they're sprouts), and also move your light in closer if it's artificial, and put an oscillating fan on it, breezing it slightly but not blowing it over...that'll make it thicken its stems and get some lateral strength...the fan tip comes from torog, and it works because it worked for me i say goddamn