It's not as complicated as you're imagining.You or your partner simply hold the base of the condom as he withdraws and as he re-enters.
In terms of slipping, it's a good idea now and then just to reach down and check the condom is still in place. Again, it's not a big deal.
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Heather Corinna
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson