I generally start with the hardscape, usually driftwood. Make sure that whether you are using rocks or wood you have good sizes to work with. You want to have some that will reach above halfway up the height of the tank. Smaller pieces while helpful in creating structure, tend to just get swallowed up by the plants. I work with it until I find an arrangement that I like. Some things that help with this are creating angles and making sure that you don't divide the tank evenly in half.
Next you want to make sure that you select plants appropriately sized for the tank. Amazon Swords don't belong in 10 gallon tanks. Try to get some variety in the leaf shape, size, and color. I then plant these around the hardscape as appropriate for background, midground, and foreground. Always make sure to plant enough of a single plant together that it feels like it's supposed to be there. Generally most plants will only get planted in one spot, maybe two. I try to put contrasting leaf shape, size, color next to eachother. I also make an effort to emphsize the hardscape in some areas and play it down in others with the planting.
I really don't do a lot of planning ahead, although I might have an overall look that I'm aiming for on occasion. As a trained artist, I rely on my eyes to tell me what is working and what I need to adjust. I find that I'm pretty good at putting together an initial scape, but still need more practice planning for the growth of the plants I'm dealing with.