The chapter's closing verse, and the answer to verse 30 rather than a separate saying.
Quick answer
Do the preparation. Then know that the preparation is not what saves you. The verse holds both halves without letting go of either.
Read both lines
The horse is prepared, stated as a fact and not as a question, which is the detail most quotations of this verse lose. Nobody is told to skip the work. War horses were the most expensive military asset an ancient kingdom could field, and the verse assumes the reader has done everything a reasonable person does. Then the second line relocates the outcome, and the Hebrew is emphatic in its word order: to the LORD belongs the deliverance. Paired with verse 30, the chapter ends by saying that neither the best thinking nor the best equipment is the decisive thing, which is a strange note of relief rather than of futility.
What it does not mean
This is not an argument against preparation, and it is regularly used as one. The verse's own first line grants the horse, so anyone quoting it to discourage planning, saving, training, or seeking medical care has cut the sentence in half. Nor is it a formula in the other direction, as though preparing well obliges God to deliver. It refuses to make either the preparation or the outcome into a lever. The book's counsel throughout is to plan, to take advice, to wage war by wise counsel, and this closing verse does not withdraw any of it. It simply says who the deliverance belongs to, and lets the reader go on saddling the horse.
Related: Proverbs 21:30 · Psalm 20:7 · Proverbs 20:18 · Proverbs 16:9
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.