Approach to allocating netblocks

For the first time we have our own ARIN-assigned netblocks that we can now split out and divide to our customers. What's the best approach to handing out /30's, /29's, etc. that is efficient as possible but allows for customers to expand their allocation to a neighboring block? I was thinking of having one /24 for each block size, and then do the divide and conquer approach by allocating the first /30, for example, as 0 and 128, then next two at 64 and 192, etc. Once there's only one /30 free between each allocation, I would start using another /24. Of course, that would mean 50% (or less) utilization. Ideas? Frank