If that were true, every vague fantasy we had in our heads at the moment of orgasm would come true within months. Intent is what makes the difference here. Forget the wanking for just one moment if you can and remember that the sigil is the important part of the magic being performed here. The moment of orgasm will clear your mind, that s all. Dancing or spinning to exhaustion is very effective.
Meditation is effective but takes years to learn properly.
Fear and shock are very good for charging sigils, so you could probably watch a scary movie and launch your sigil at the bit where the hero s head comes bouncing down the aluminium stepladder into his girlfriend s lap. A run around the block clutching a sigil might be enough to charge it. In spite of His thoughtful addition of a clitoris to Adam s lively rib God apparently overlooked the potential for female masturbation and forgot to prohibit it. Under these circumstances, I d suggest a chaste kiss or a ride on the dodgems, either of which should be enough to white out consciousness for as long as you need. Bungee off a bridge, perhaps, or sit naked in your local graveyard at night.