Try grabbing onto the arm. If you pull the arm out like an armbar it keeps them from being able to tense up alot of their body and it gives you more control over them. The strengths of the legs and tendons in the groin area take alot of conditioning before they can just lock down a solid traingle, but the arm thing helps.
Also, don't just limit yourself to a figure four, tighten it up by moving the leg that is against their neck farther up your leg until it is at your knee, then you can strighten out your legs. This really helps to maximize the power of the choke, at least that's what I've found.
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Barnett - Toquinho -Werdum - "Nurmie"
Z. Gurgel - Morango - Rocha - Tiequan