I did not really use stitch markers until I was in my 40s. I’m sure it caused problems, but I wasn’t aware, so didn’t know that markers might have solved the issues.

Favorite Stitch Markers and Where I Use Them
I will use anything, but I have preferences. I love those rings with the beads. I think I have thrown out those fat ring markers because they created spaces. I still like the basic think rubber rings, but I don’t have a great selection anymore, so when I have to use a lot of markers, they get left behind.
What do I mean by a lot of markers?
Well I often put a marker in to mark off each stitch repeat. Right now I’m making something with 21 stitch multiples. I have a BOR marker, plus 20 others. I need to be able to read subtitles while knitting for many of the programs we watch, and I like to be able to see my mistakes quickly.
I bought those large round metal ones, but I don’t really like them. I now have hexagonal markers that size. They sit better on my needles, although they can be a little poke-y. I am generally knitting on US size 2(2.75mm) to US 5(3.75mm) needles.
See the ones with the big beads? I can only use those for BOR markers. I find them awkward. There is one from that set that has a dangle on it. I do not like dangles. They are so cute, but they just don’t suit my manner of knitting. Sometimes I use one as a BOR if I am missing it–the annoyance makes me pay attention.
I like pin markers when I’m counting cast on stitches for the counting reason: they go in every 20 stitches, but sometimes I can’t count, so the pin allows me to move it if I discover a mistake. I am short on the orange and aqua pins, and I have other similar ones, but they are hard to undo so not my favorite.
My other preference for the pins are the ones with jewels on the pin. The jewel weights them and they sit nicely on the needles. The others, many of which come from clothing tags, flip around a lot. The company I got the preferred kind from isn’t around anymore. I’ll be looking for a substitute at H+H
GSR markers
I’ve recently begun putting a pin marker in to mark my GSRs. Jamie and I love when we can write the instruction to “work to the GSR” and as I’ve been doing these in the round, it is so nice to just have a pin there to tell me the next stitch will be my GSR. The only stitch counting I have to do is from the last GSR to the next one, place my marker and “work to the GSR” on the other side. Whoot. Makes for easy knitting.
Before we get on to other marker uses, I need to confess that I love to knit. But once I’m on a project, I really want to make progress. So markers let me knit towards my goals.
Marking the Fabric
Visual Reference and Progress
If I need to work from the RS and the WS, I place a RS marker for quick visual reference. Likewise, if working with dpns or doing magic loop, I put a pin marker in the fabric to mark the BOR.
Sometimes I use pin markers to mark my progress. For some reason, knowing I accomplished something concrete always makes it easier to proceed.

Tip for Counting
Here is a tip for marking out row numbers. I know a lot of you mark every X rows/rounds with pins. This is how I do it and then your progress markers won’t be confused with counting markers.
Measure out a length of contrasting waste yarn that is longer than the total length you will be knitting. Put a slip knot in one end. Slip knot the other end together with a yarn tail at the beginning. Now put the other slip knot on your knitting needle (in the round it works as a BOR marker, or a half-way point marker) and *slip it every row/round for 10 rows/rounds (or your preferred number). After 10 rounds, slip it but bring it so the yarn will run up the opposite RS/WS of the knitting. Repeat from *. You now have a running marker that is in your knitting so you don’t have to put your knitting down to mark.
This yarn length marker can be useful for other things too. I have done it on a steek–then the fold is marked — but doing it every other row so it is like basting thread. It could be used to mark pocket placement, or other detail. Just loop the slip knot over an adjacent stitch when you are finished marking.

Marking Out Shaping
Decreasing
When I am working decrease shaping I put in marker(s) to mark how many stitches I have to remove. Then I work along in the decrease sequence until I am to the marker. I may need to do the last decrease with the stitches on both sides of the marker to complete.
Increasing
Working increases I am creating stitches, so I have to do a bit of counting. I will place marker(s) where the increase will be happening and write down how many stitches need to end up between the marker(s). That means that I can work the increase sequence only having to keep an eye on how many new stitches have appeared.
Evenly Divided
If working say evenly placed increases, I likely will put in markers (if it is a large number of stitches between) so I can work to each marker and increase or decrease as needed.
Markers allow my knitting to have more flow. I can work without having to focus on a single thing and instead enjoy the yarn going through my fingers, the stitch pattern, how the fabric is developing, and oh, yeah, what is happening on screen!
The aid of markers make my knitting time productive and enjoyable.

Thanks for the idea of using contrast thread vertically as a means of counting rows! I’ve never heard of that before. Clever and simple.