A pair of well-fitted skinny jeans are essential, I often wear black canvas since they are a bit stiffer and have a nice drape... they can also look like a less-formal suit when worn with an equally slim blazer. I have got two pairs that I wear all the time, one from Dior Homme (black toile jeans from S/S 2005) and one from ACNE (Hex Cash, women´s skinny fit)...
Since I often work in layers, tank tops are essential, too - mostly in neutral shades like black, grey, beige or khaki with white trim.
Last but not least, a pair of good old, minimalist sneakers, such as Converse or German army.