I am going to give you different advice depending on whether this is an anal or a vaginal toy. Anal seems a lot more damage-prone to me, and so I'm not one to advise pushing large things through in one session. However, with vaginal toys, the feeling of holy-shit-out-of-contr ol! stretching can be part of the fun.

Before I successfully tried a very large vaginal toy, perineal massage helped me prepare. You can google this: it's what some practitioners tell pregnant women to practice prior to vaginal deliveries. You don't have to go overboard, but stretching yourself in the perineal direction with your fingers first means that the toy is less likely to put all of its pressure on your more delicate vestibule. My post-penetration inflammation went away when I started doing occasional perineal massage.