Approach to allocating netblocks

[frnkblk at iname.com (Frank Bulk)]

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