Increases and decreases can be done in several ways, and they don't all look the same. Choose one and stick to it for each project!
The picture shows 3 x 3 different increases, from bottom to top: loop, running thread and row below.
(Paired) decreases
Slant right (on right side)
On knit: K2tog
On purl: P2tog
Slant left (on right side)
On knit: SSK *
On purl: SSP TBL (slip 2 K-wise, return to left, purl though back loop).
Link to Amy Herzog waist shaping article. Basically, the decs should lean towards the marker.
*An SSK is usually rather visible, because of the way the stitch is formed. The stitch "on top" becomes rather loose. The best way to avoid this, is to plan all decreases on the purl side. The SSP-tbl is practically invisible on the knit-side. If you have to do an SSK on the knit side, use Grumpy's (Grumpy Grandma on Knitting Help) method:
- slip 1 knitwise
- slip 1 purlwise TBL
- k2tog TBL
Increases
The loop, as advocated by Elizabeth Zimmerman, is just that: backward loop on the needle. Very simple, and I've used it a lot, but it's not the prettiest increase.
Running thread
Slant right (on right side)
On knit: pick up strand from front, knit through back;
On purl: pick up strand from front and purl through back.
Slant left (on right side)
On knit: pick up strand from back, knit;
On purl: pick up strand from back, purl.
Row below
Slant right (on right side)
On knit: knit into st below, then into st;
On purl: purl, do not drop, purl into st below.
Slant left (on right side)
On knit: knit, don’t drop, knit into st below;
On purl: purl first into st below, then into st.