A sim who knows 25 others already achieves the Celebrity Charisma skill challenge and will automatically get a relationship bonus when meeting someone the same goes for a sim with the Attractive lifetime reward (15,000 points). It's not even difficult to make so many friends either. I believe that a sim does need to learn the skill in order to get the benefit, but one point is enough. This is a bonus from finishing a challenge associated with the Charisma skill. If your sim makes 20 friends, those friendships will never decay on their own. The other way to keep friendships from decaying is through the game itself.
I've never used any of them though, so I can't tell you for sure how well they work. There are several out there, depending on what 'flavor' of slower or lack of decay you want. To stop the decay by default though, you'd need a mod.