When your dog is a mixed breed, you have to consider the features and hair type in order to make a style. Many times, you can take the styles of a few different breeds in order to make a unique trim for your mixed breed. Maybe a Poodle head with a Terrier-type body and making the legs into bell bottoms would work well.
Depending on the dog, you may have wiry hair on the back, soft hair on the legs, and curly hair on the top of the head, with a long tail that's thick and bushy like a Collie. Go with the dog's hair type and work with it as best you can. If it is too hard to find a style you like, you can always shave it short and leave the parts you like. Maybe leave the long tail and ears and take the rest of the body short, leaving a little eyebrow or moustache for personality.
Enhancing the dog's best features gives the dog his look. Eyebrows make the dog's eyes appear as a strong feature. You want to give them a little curve and make the inside corner of the brow longer than the outside. You can have rather short eyebrows but leave longer hair on the inside, going down toward the nose, keeping the vision line open but creating a unique look.
If your dog has a beautiful tail, brush it out and trim it lightly, and trim the body shorter to make the tail stand out. If your dog has long, beautiful ears, leave them long and trim the neck and head of the dog shorter to show off those gorgeous ears! Most people want to make their dog's face the focal point; after all, who could resist those doggie eyes?