Some knitters find that their tension, when working lace, is looser than it would normally be. To address this, they are often advised to go down in needle size for lace.
Personally, I do not agree with this advice - because it addresses the problem only when a project is either entirely done in lace, or when the lace presents itself in horizontal panels.
But what happens when the lace appears in vertical panels, so that in the same round/ row you must work a section of stockinette punctuated by lacework? You will now have to use the same needle size for both sections. And if you have never learned to streamline your tension across platforms, as it were, your lace sections will look sloppy and overly loose, in proportion to the stockinette fabric - which is especially problematic when the vertical lace panels are in areas where they take a lot of stress.
For example, one of the garments I am currently working on (top of the photo) is a fingering-weight version of Pale Fire (shown in Babbles Yarns). There, you can see that the lace is used as a decorative element along the raglan line, which means the entire sweater is basically hanging off the lacework. Now, I deliberately used a motif here that is only slightly lace-like and really has shown itself to be quite stable. Still, you don't want your tension to be loose in the part of the sweater that gets the most stress, causing the raglans to spread uncontrollably.
The other project I am working on (bottom of the photo) is a cardigan with vertical lace panels alongside the button bands (shown in Apple Oak Fibre Works yarn). These panels, while not structurally significant per se, will be stressed whenever the wearer tugs at the button band. If the lace is knit too loosely, it will quickly distort out of shape.
So how does one keep their tension tight when knitting lace?
According to my observations from watching knitters struggle with this issue, the main culprit of the looseness is the yarn-over (YO). So my suggestion would be, to pay attention to how you are executing that step, and find a way to tighten it up - to release less Y when doing the O, as it were.
More specifically: Observe your hand movement when doing a yarn-over. A flamboyant flick of the wrist can result in an over-generosity of micro-millimeterage bestowed upon the needle. Rather than wrapping yarn over the needle, treat the YO almost as if it were a slip stitch. Tighten your hand movement and let the needle only just catch the yarn on its way to the next stitch.
You can practice this in slow motion, until it becomes natural. And trust me, it won't take that long until it does. Being able to hold your tension consistent as you switch between stockinette and lace is a useful skill, for which your lace-paneled garments will thank you!