As a general strategy guide, I have some things to add to this, too:
1. As Samson, treat him as you would a Scout from TF2. If you don't play TF2, basically, never stop moving, and don't back yourself up too much. Keep moving and peppering on the damage; the more you do, the more you get, so even if it's a slow process, it's safer than taking a couple of hits to try and charge it up slightly faster. Stay on the move at all times, and try not to charge at enemies like you would with someone like Judas.
2. Number One + any range increase modification is basically a sweeper tool for Samson, as Espurr said. Alternatively, anything that makes you shoot faster is good as well, and doesn't pack on the range decrease that Number One does, helping you stay on the move. If you have good range modifiers BEFORE picking up Number One, I'd simply recommend waiting for something better. After all, the key to playing Samson is to stay safe.
3. Alternatively to Brimstone, which fires just a bit too slow for my taste (though for the record, is a good suggestion for an item), Polyphemus is an incredible tool that, despite being slightly harder to land hits with than Brimstone (unless you're more used to it than Brimstone, like I am), packs on damage indescribably quickly and really loves the boost from Rage.
4. [Haven't tested personally] Anything that spawns flies or orbits around you that can do damage quickly, safely, and passively is good for Samson too, to pack on a bit of damage while escaping or circling an enemy without having to fire. Certain flies are stronger than others, though generally the further out it is, the better.
5. A risky maneuver could be to gather items that decrease damage but greatly increase firing speed. While damage will be put on slower, the more you build up, the less it matters that you decreased damage, and Samson will greatly appreciate the faster firing speed. However, if you're not a survivalist, you may be better off skipping items like this.