Ok, here goes;
Sittups- just do it twice as slow... by doing that ur excerting more power and putting more effort onto the muscles... the slower you do it the harder it gets... bend ur knees (like ud ussually do), cross ur hands over ur chest, and dont forget; if u dont want a stiff neck by the age of 25... then the trick is always to look at the sealing... trust me...
rotate the positioning of ur legs so that you could work on other parts of ur abs...
Pushups- Correspondingly, do it slow for a better effect... and if u wanna change the resistence just move the positioning of ur hands... the closer they are to eachother, the harder it is... no need to place ur feet on a chair... it wont make much of a difference... and, nevertheless, its not good for u as the blood flows up to ur head which may result in a migrain...
You wanna build muscles as quick as possible..? gain strength...? eat 30g of protein b4 each workout...(eggs, fish, meat, shakes, etc...). Likewise, you could also use amino acid pills... amino acid is essential in building and repairing muscle tissues... im not gonna get into all this crap now coz i dont have time... most of what i said is probably obvious, but i thought it wouldnt hurt reminding u...
Cya later muscle boy...
IrReSiStIbLe.