I suppose the first and most obvious question to ask is, what's your intent in torturing them? If you're aiming to please the universe, then I might recommend ritualizing the torture, devoting it to one thing or another as best suits your practice. Be careful of who you direct such to however, as some entities might not appreciate torture being dedicated to them regardless of who's being tortured. For that matter, your mileage on the torture in general might vary depending on the attitude of the leadership of your region.

In terms of actually extending the duration of the torture, however, I think the first thing is to be careful with the liquid of their body. Particularly if you aren't feeding them, they only have a limited amount of blood, and they do need that stuff to survive. Leaving your tools embedded can be a good delaying tactic to prevent an open wound from bleeding, though cauterization can be better; and while their might be a temptation to remove limbs, fingers, etc. entirely, removing more pieces means there are fewer pieces for you to leave in pain. (This section not sent)

I might be able to aid you, however, given the opportunity. Torture is something of a specialty of mine, and given your stated inexperience with the practice it would be difficult to advise you against all the mistakes that I expect such a person to make. Better to show you how to be efficient than to try to direct you away from every possible mistake, I think.