There are no really hard and fast RULES per-se, but there are a lot of example artifacts and they follow a pretty structured pattern of design. I've created a number of them and its really no harder than creating a monster from scratch.
The easy way is take an existing artifact or two that have similarities to what you want, DMG2 and some of the supplements have a pretty wide variety of examples all told (the Compendium makes it simpler). For weapon-like artifacts a good idea is to start with a base weapon enchantment and weapon and then build from there, looking at existing artifact weapons for cues. You can build something playable pretty easily. The fluff, personality, and any "out of the box" sorts of aspects of the artifact are always the trickiest part anyway. That's really just going to depend on pure DM creative talent and gut instinct.