Nonograms are all about logical deduction. You should never at any point feel the need to guess, as there should always be a next move suggested by the current state of the puzzle.
While you may not have all of the clues to fill in all of the squares in a row or column, there are some squares that will be filled in any case. Fill those in right away.
Use the X marker to rule out squares that you know won’t be filled in. Even if you’re an advanced player, you might still overlook something if you aren’t diligent about this.
Go for the big numbers first. They’re always the easiest lines to complete, and they’ll give you a leg up for figuring out the rest.
Sometimes you have to think about what isn’t filled in over what is. While you can’t always determine which squares to fill, you can probably rule some out. Plop down those X markers.
If you make a mistake somewhere along the line and end up with an incorrect solution, you have a couple of options. While you can erase squares and try to backtrack to a correct state, this is actually one of those moments where you should try to see the picture you’re making and see if you can straighten it out that way.