1. Pastels can be very messy and if you use your fingers even just a little bit...your fingernails are always stained with paint. I try scrubbing them clean with an old toothbrush and that helps a bit.
To eliminate the problem this is what I'm doing now. I bought small latex gloves (which were too big) and small finger cots. The "finger cots" you see in this photo were too tight and quickly cut off circulation. I hated to throw them out...so I unrolled one, cut the excess latex off and put it back on...the problem was solved. Now they are very comfortable and will save my poor fingernails going forward. You'll also notice I have backup finger cots already cut down and placed over pens/pencils in my pencil jar ready for use. I rarely need to change them during a painting because I keep wet wipes on hand to clean them along the way.
2. If you use an iPad or similar device for your reference photo: when sketching the image select "edit" and "crop" to view the cropping grid superimposed on the image this helps properly scale the subject matter.
Also, after the sketch is done use the "tonal filter" to convert the image to gray scale...this helps select the proper values when applying the under painting.
3. Rita Kirkman gave a great tip on cleaning pastels with toilet paper. I am also using it when selecting my color palette. It cleans the pastel and at the same time gives you a good look at the colors you've selected. If you don't like a particular color, just mark it out with a magic marker. I also lay out the colors on my palette in the same order as they appear on the TP.