Try to use objects that are thin and easily wedged between your page and the glass sheet. The less room there is for your print to move and the closer your print is wedged between the glass sheet and page you wish to print on, the “cleaner” the printing outlines of the object will be. On botanicals with thick stems, you can shave down the thickness of the stem, retaining the shape, but making for better/ closer contact with the page. Any room for light to get in, will affect the print. This can however be used as a method to get different shades of blue in your print should you desire to do so.
You can also add a small amount of white household vinegar to clean rinsing water (After the print has been rinsed thoroughly to remove all the chemicals), this will brighten the blue and has proven to assist in defining the print slightly, especially in the case of using Brown Ferric ammonium citrate.