One of the best ways to make sure you get into your field is to establish a defined set of measurements. Here is where high and low key points can really help you to adjust your plan.
In a PA28, the correct spacing occurs when the touchdown point is approx. in line with the 2nd rivet line in from the wing tip (most likely the same method you may be using in the circuit). If you arrive at the high key point and the touchdown point is too close or too far away (identified by looking at the relationship between the rivet line and the touchdown point), a simple correction of either widening the turn to the low key point, or tracking direct to the low key point can be made. Combinations of height and spacing need to be considered.
You can use the same process during the turn from downwind onto finals. By making small heading changes (10-20 deg) then flying wings level and allowing the leading edge of the wing ‘intercept’ the touchdown point, you can make an assessment on your spacing. Basically, make the heading change, fly wings level, then make an assessment, correction (i.e. delay the next turn, or turn towards the field) then make an evaluation. It is a very similar process to flying a DME arc. The whole way throughout the plan you have a way of identifying what corrective action (if any) is required.
Like Oktas8 said, make certain that flap is not selected until you can bet your life that you will still be able to make the field. A habit that people can fall into is selecting 2 stages of flap in one go. If they have only flown circuits previously, then the action of automatically taking 2 stages (eg. turning base) can be instinctive.
It is a bit hard to explain in text, but I hope this helps nonetheless