Regarding the Cellphones, I only bought the one in Rose Gold, because it reminds me of an actual cellphone you could see irl, but since I don't make such every-day-real-life avatars, I only got that one and don't intend to buy more of them. I think I only used it once, right after I bought it, and that's it XD
My strategy with Commons is juts like Amura's, I buy things when I need them. Unless I know for sure I'll need that item in all possible colors, that's usually for frames and borders, maybe some accessories, rarely clothes (unless it's Bananarama, 'cause that's cheap and won't make me feel poor after I spend 30k on a dress

)
I stopped collecting all possible monthly troth items, I don't use them most of the time, even when I like them ;/ I onlybuy the ones I like and might use, but I always mentally prepare myself for not ever using them xD