Other Materials and Equipment
All the items below are useful to the artist but like most things there's a wide choice of products/brands.
There are a lot of so called 'useful' or 'must have' products on the market but all that is needed are the basics below, or to put it another way 'keep it simple and don't waste your money'.
Pentel P203 and P205 Clutch Pencil and Berol 'Venus' Pencil
A sharp 2H or HB pencil is all that is required to draw in the lines of the subject you are going to paint. In the studio I use two Pentel Clutch Pencils, one containing 0.3mm and the other 0.5mm leads, usually 2H or HB if drawn lightly.

The Pentel Automatic Pencil 0.5mm and Clutch Pencil 0.3mm are suitable for drawing and designing. Each gives precise, clean and accurate lines. Many artists swear by clutch pencils. Timber-cased pencils change their size, weight and balance as they are sharpened, which can be a problem for artists who draw a great deal. Clutch pencils have a constant weight and size and though initially expensive, the refills are competitive (packs of 12 leads).

When I'm sketching outdoors I use normal pencils (less expensive if lost, easy to do in long grass or on the beach and less prone to damage). I sharpen pencils (Venus from Berol are the pencils I have used for many years) with a sharp knife (if you can use a knife for sharpening, you will save a lot of wastage compared to a sharpener).
How to sharpen a pencil using a sharp knife:
Hold the pencil with the tip towards the ground.

Hold the knife blade at a 45-degree angle from the tip of the pencil.

Begin at the lip (where exposed wood meets painted edge). Press firmly but lightly (going from the pencils painted edge towards the graphite), take one shaving from just the wood (not the graphite). Rotate pencil 90-degrees and do another shaving. Repeat rotations and take thin shavings until the required length of graphite is exposed.

Finish by sharpening the graphite itself. I like a longer point so when the pencil is held almost flat to the paper so a larger area is shaded but is more likely to break.
Winsor & Newton Colourless Art Masking Fluid for Watercolour
When required to mask areas of my paintings I use Winsor & Newton Colourless Masking Fluid. This is applied with an 'old' Winsor & Newton Series 7 Kolinsky Sable Brush No 6 dedicated to this purpose.

To keep the brush clean and workable the following process needs to done (see below) on every application of the masking fluid. This process has been used on the same brush for almost two years but please do not use 'new' sable brushes (many artists recommend using cheaper brushes and would never use a sable) for masking fluid, the one I'm using had done a number of years work before being used for masking fluid.

I cut the brush tuft (from an 'old' No 6 brush) in half, across the diameter (not the length) which provides a nice length of hair, to carry the masking fluid, but also comes to a nice point making it ideal for painting (masking) thin lines.
How to keep your watercolour brush usable when using it with masking fluid:
Note: Between applications of masking fluid the brush 'tuft' is covered with soap. This needs to be removed before using again. Also make sure that the paper is 100% dry before using the masking fluid.

First clean off all the soap (from last use) in the water pot used just for cleaning the masking fluid brush.
The dry any access water from the brush using a paper towel, so as not to dilute the first application of masking fluid.

Open the jar of masking fluid and working as quickly as possible apply the fluid to the required areas. If a large amount needs to be covered and/or more than about 10-20 minutes passes or the masking fluid is beginning to dry on the brush, stop and clean the brush (as detailed below) and start again.

When finished painting on the masking fluid, close the jar of masking fluid and quickly dip the brush 'tuft' into the pot of water. Some masking fluid may well have started to dry on the brush and this is removed by brushing the tuft of the brush on a bar of soap. A helping hand is usually needed by using your nails and fingers to remove all traces of the drying masking fluid. Apply more soap if needed but all traces must to be removed.

When clean, use the tuft of the brush to lather some soap and cover the brush tuft with the lather. Place the brush flat to dry and ready for next time.

Make sure the lid of the masking fluid it tight and then turn the jar upside down as I've found this helps slow down the air getting into the jar. Masking fluid does go off and if it goes more solid and unworkable it's time to replace.
Water and Water Pots
I recommend the use of at least two or more pots/containers/jars. Personally I use three pots for water.

Two of the pots are used when painting - one has clean water (clean washes and mixing paint) and the other one for dirty (washing brushes).
The third water pot is used only for cleaning the masking fluid brush, as detailed above.

Change the water in the pots often to avoid the water becoming to muddy.
Other useful items include: Ruler, Knife, Cutting Mat, and Rubber.
Ruler:
A metal 1m ruler or straight edge is preferred but a plastic version can be used (being careful when cutting along the edge so as not to damage it). Always cut on the waste side.
Knife:
Swan Morton, available in most art/craft/graphic shops are ideal. They have different shaped handles (No. 3 and No.7 handles are the ones I use) and a selection of blade shapes are available. I've found the No. 10A Swan Morton Blade (straight cutting edge) to be the most useful but also use No. 10 (which has a curved cutting edge) while No. 11 is a more tapered version of 10A. Other blade shapes are available.
Rubber:
Not used very often as drawing another line is the best anyway. If you need to use a rubber its best to wait until the painting is finished and then remove the marks so as not to damage the surface of the paper. As a side note some people like to see the pencil marks or drawings on watercolour paintings. I would recommend putty rubbers which are better than harder rubbers.
Cutting Mat:
Save the table etc by using a cutting mat. The self healing versions of Cutting Mats, though more expensive, do last longer and over time are better for cutting on, as the non self healing ones with repeated cutting small sections of the surface break away. Price varies with size.