Well this is very much dependant upon what you’re posting and how many but, leaving that to one side for the moment, let’s start at the beginning: who you’re mailing to.
Databases can resemble an out of control garden sometimes: things in the wrong place, a lack of structure, and, sadly, the deceased still in place. If you’ve a number of people inputting data make sure they’re following the same rules and use places like us or  a data bureau to keep postcodes current and accurate as well as screening for gone-aways etc.

Timing can certainly keep costs down. If you’ve left yourself plenty of time for a sales brochure (for instance) why not take advantage of Economy services rather than 1st or 2nd class. We come across a significant number of people who have been tasked with organising a mailing but think that there’s only 1st or 2nd, so if they say to us “…and just send it 2nd class” we have to ask “Do you mean actually 2nd class or ‘cheapest’, in which case…”

Now let’s look at volumes. So for Letter size the minimum quantity for accessing Mailsort tariffs is 4,000 and for Large Letter it’s 1,000 items. If it’s below these limits then people sometimes assume that it’s the same as the unsorted stuff they put through their franking machines. Not so. With the Royal Mail Online Business Account there are volume related discounts to be had. If you have 500 or more there are also machine readable discounts, such as OCR. You basically start at the franking machine price and then start stacking up the discounts. So let’s say you have a car dealership (lucky you) and want to advertise an event day (such as the launch of a new model) and have, say, 1,300 existing customers and prospects you’d like to invite. The mailing piece you’ve designed is an A5 invite and an A4 letter which you want inserted into a C5 envelope, therefore “Letter” size by Royal Mail specifications. If you used the account tariffs instead of a franking machine then you’ll have the volume related discount. If you’ve also qualified for OCR (or bar code) then you’ll have the machine readable discounts also. Then there’s the fact that you’ve got more than 1,000 items and you’re advertising an event so you qualify for the Advertising Mail discount. If you’ve got the quantities for the Mailsorted options you can still have access to volume related discounts, machine readable discounts, and Advertising Mail discounts (and that’s on top of the Mailsort discounts). I should perhaps point out that I used the phrase “You basically start at the franking machine price” but at the time of writing Royal Mail have introduced a tariff called Account Barcode Unsorted which starts at a penny per item below the regular unsorted prices for OBA and franking machines. This of course requires a supplier who can furnish you with the correct bar code Postage Paid Impression (hello there).
There are other avenues open to you to try and squeeze as much out of your postage spend as possible (eg DownStream Access options) but I see the word count mounting and am aware that you’re likely to be getting a bit bored with my dry prose. So I’ll leave you with some obvious advice: if you’re about to plan a mailing talk it through with your friendly Mailing House or Royal Mail account manager or perhaps you know someone who has experience of bulk mailing. Pick their brains and review all your options because you probably have more than you expect.