Ian, I'm pretty sure when I first started I was the same, however each and every job was immaculate.
You'll get faster with practise, but if you can, try and find a friendly local window cleaner, (someone who looks like they're doing well) who'll take you out for a day and show you the ropes. This could be worth £1000 of pounds in real terms since you'll learn how to work faster and price without making too many mistakes.