Running stitch is not a difficult stitch to learn, perhaps it's even one of the easiest stitches, but it takes a little bit of practice to get a (nearly) perfect running stitch. Over the years I learned a few tips & tricks that'll help you get a great running stitch every time.
First of all, try to do your running stitch without a hoop. I nearly always use hoops and frames for my embroidery work but with the running stitch I find it's easier to go 'hoopless'.
Choose a slightly longer needle, like a milliner's needle for example (see photo) you will understand why when you see my running stitch technique.
Use a line on your fabric as guide. This can be an erasable continuous line (use a water soluble marker for example) or, if you prefer, mark your fabric with dashes, in the length of your stitches.
Running stitch as shown in this tutorial is made using the 'sewing method'. The idea is to weave your needle through the fabric, making several stitches at the time. The alternative is the 'stabbing method' where you pull the thread through with every stitch.
Start by bringing your needle up as shown in the photo above (top left). I embroider left handed so if you are a right handed stitcher you would probably start on the other side.
Enter the fabric with your needle a little to the right (or to the left if you started on the righthand side), but do not pull through. Instead weave your needle through the fabric and bring your needle up about a stitch-length away (top right). Repeat a few times ( as shown in photos at bottom left and right). How many times you can repeat the 'weaving' without pulling the thread through the fabric, depends on experience (or talent) and the length of your needle!
To finish, gently pull the needle and thread through the fabric. Take care not to pull too hard! The result is a straight and even running stitch.
A simple and very decorative variation using running stitch is to combine horizontal and vertical running stitches into rows of 'plusses'
Start by embroidering several (horizontal) rows of running stitches. Then turn the fabric 90 degrees so the stitched lines are vertical.
Embroider as much rows of running stitches as needed, making sure to cross the stitches embroidered previously to create 'plusses'.
To the left a few other running stitch variations. By shiftings rows, varying spacing between stitches or using different colours, many different effects can be achieved.
Hope you've enjoyed exploring running stitch with me!