How can I reduce sillies while in programming contests ?

For example, in the last contest I took in some other coding site, I solved (solved means got the idea, just need to implement them) FIVE questions within twenty mintues (out of two hour contest). Now the first two was completely trivial to implement — I did that within ten minutes. So half hour, two questions down and I only need to implement four — sounds trivial right ? But I couldn't successfully implement any of them ! In one question I forgot to use long long instead of int, in another I forgot to increment by one in a case (but I didn't test that case), and in another question I couldn't figure out how to implement them (it looked very difficult).

So some questions:

• How do I reduce the excessive amount of sillies I make while implementing ?
• How do I implement effortlessly ? (In ideal world, implementation shouldn't take much thought right ?)
• How do stop losing my sweat over those edge cases/off by one errors ?