if you're filling the bucket of suds by tap then vigourously agitate the water as you fill it
You don't need to go after lots of suds to clean the car - you just need to make sure it's adequately mixed. I always pour the shampoo into the bucket first and then fill it with water. Usually enough to get an adequate mix.
Like I said, you're not after suds, just a good mix of the shampoo and water.