One the easiest things to do when lifting is to develop bad habits. It takes knowledge, concentration, and coaching to lift correctly and achieve the optimal results. As such, one of the most common mistakes in any lift, albeit squat, deadlift, push press, any form of row, or even planks is a rounded spine. This can only lead to trouble.
When rounding the spine you pinch the anterior aspect of your spinal discs together, over time this can result in a protruding disc or degeneration of cartilage between discs. As well as creating muscle imbalances from not keeping strict form. For instance, if your back squatting and forget to keep your low back contracted and you lumbar spine rounds out you can cause a anterior pelvic tilt that will knock your hips out of line and cause lower back pain.
The first step is to learn how to feel for a neutral spine. This can be done by getting onto all fours and doing what are known as “cat-camels”. Where you contract and extend the spine past neutral in either direction and then settle for the middle or neutral position, often you would have an observer tell you when you hit the mark. Once you have gotten comfortable with feeling this position you apply it to your lifts, always beginning and ending your lift with that neutral position. It is always help to lift with a coach, someone who can spot your form and help you make adjustments. If a coach is not available then you can always settle for a mirror.