To caste a sigil effectively, use a method of creation for the sigil that suits your mentality best, and be very serious about the sigil; think it over if you must and hesitate, wait until you are prepared to accept fully the karma of the sigil, then caste it with the fixed belief and perhaps feeling of horror or excitation that it now can not be undone.
You must be of a mindset that, whether you like it or not, whether you change your mind or not, the sigil once casted will take hold. You must give no resistance whatever to it in your intentions, you will carry it through, even if it be to your great loss and destruction.
Then, no matter what you think or feel thereafter, everything simply serves to empower the sigil.
It should be designed out of words or a formula that satisfies the various elemental needs and has a definite time variable, although it doesn't have to manifest at a specific time, there needs to be a sense of it manifesting over a period of time in relation to the emotional input and the difficulty of manifestation.
It should be specific enough but also open enough to allow reality then to work your request into manifestation.
I don't destroy my sigils until they are done. I leave them and charge them whenever, feeling every thought and emotion leading to one conclusion; the manifestation of the intention behind the sigil. You can take any feeling and make it work to charge it by feeling that all your thoughts and feelings are doing that, to your dismay or pleasure, annoyance or panic, etc.
The rest is a matter the three laws of manifestation and you can get others to charge your sigils inadvertently too.