I discovered recently that the word 'straight' in this verse means clear, or obvious. That is, His will for our lives will be as obvious as the grass is green. But why doesn't it seem that obvious? Let's look at the first part of this passage. "Trust in the Lord with all your heart.." Most likely I'm not completely trusting in him. This kind of trust says 'YES!' before even knowing what will be asked. I think many times we say to God "Lord show me your will so I can consider it." And then we wonder why there is silence from Him. That's not the way He plays it.
"...don't lean on your own understanding" - Say yes and don't 'weigh pros and cons'
"...in all your ways acknowledge Him" - don't compartmentalize your life. Make Jesus Lord of everything. Relationships, work, school, family.
And HE will make your path clear and obvious.