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Using Crop Grids and Setting Key Images
A few weeks ago I mentioned that I was working on a video tutorial series for Aperture - and because of that I occasionally come across some features of Aperture that I really hadn’t paid much attention to or was unaware of. (The video series should be available before too much longer, and I’ll let you know details when it’s available.) Here are a couple of those features.
As you probably are well aware, when the crop dialog appears you can specify whether to constrain the proportions or draw a freehand crop.
And you can flip the vertical and horizontal dimensions by clicking on the double edged arrows between the two settings. But have you tried holding down the Command key while drawing or adjusting the crop? An overlay appears with a grid marked into the rule of thirds. That makes it really easy to place the crop on your image so that a key element is on one of the intersections of the lines. Obviously you don’t always want to follow the rule of thirds, but the grid is a helpful starting place as you crop and recompose the image.
Another feature you may not be aware of is how to set the Key Photo for the All Projects view (or even what a Key Photo is!). Right under Library is the All Projects project. Click on it and thumbnails of all your projects appear. 
When you move your cursor over any of the project thumbnails, you can scan through the images that are within the project, The image that you first see when the cursor is not hovering over the project, is the Key Photo. To set the image that that appears as the representative image for the project, open the project as you normally would, select the image and go to Image > Make Key, or Control + click on it and choose Make Key Photo. By the way if you haven’t tried Control + clicking on an image, you might want to do so to see the impressive list of options you can access that way. It can be a real time saver.
I recently discovered the thirds grid myself and it made it SO much easier to crop photos. Before you had to guess and hope your were close. Now, if they would only include centerlines in the grid, we'd be set. I'm still guessing with that kind of positioning, but the handles at the edge of the crop tool help some.
'Control+click' ... just call it right-click or right mouse button..
'open the project as you normally would' ... or just double click on the project in the All Projects view (a way that would not normally be obvious for those not aware of the All Projects view)
'open the project' ... that is not necessary you can just hover a bit over the thumbnail in the All projects view. And when you see the images you want as Key photo just press the space bar, done! (or right click it right then and there and choose Make Key Photo). one tiny problem might be that when there are A LOT of photos in a project it might be hard to locate the one you want as key to show up in the all projects view.
Using a tool like the thirds grid I always fear my crops will look too mechanical. I mean, I tend to follow the rule of thirds most of the time, and I find that when I put pictures into a slideshow the difference between the focal object always staying pixel-accurate at the third-intersection and the focal object shifting around but at least moving is significant.
In any case, a good tool to have in our belts. Just make sure you're not letting the tool override your artistic eye for the right composition!
Tom, as with any tool, it's just a starting place. Use it as a guide and then go from there. Photography is a visual art and the final decision on the best composition must be visually determined. The grid isn't there to imply that every image must have the focal point precisely on one of the interesting points - sometimes it's more effective farther over and sometimes centered, etc.
Arne, yes there are other ways of doing and referring to the same process. Thank you for elaborating.
Great tips. Thanks for sharing. Aperture is such a powerful and deep application. It's easy to just use what you know and not dig deeper. I think the grids on the crop tool will definitely come in very handy!