I've bought the largest pots I can afford to allow for root growth and impact 
Agree with the evergreen shrubs idea for year round interest. I have rhododendron, fatsia, phormium and box balls interspersed with pots of trailing annuals and herbs. It's useful as you don't have to re-do them twice a year and they look nice in winter (may need a bit of fleece protection during frosts depending on where you are).
The glazed pots came from eBay locally (people moving house) and I've picked up quite a few medium plastic ones from the pound shops which I aim to gradually replace.
I have a couple of plastic flexitubs and a tower of nine stacking trio pots (look for them in the pound shops, 3 for 2). My latest purchase has been enormous plastic planters ('bell pot' style) from B&M, 3.99. They have them in terracotta and black if they're still available.
I'm a fan of vertical planting so I like to get creative with my boundaries
I have hanging baskets, hanging pouches and sweetpea wigwams in addition to my trio pot tower.
I think height definitely makes a difference in terms of impact and leads your eye around the space 
I'll admit it is a bit of effort to fill them initially and I have to re-do them twice a year apart from the evergreen shrub ones. I tend to scale things back in winter and stick to bulb lasagnas with winter pansies on top, and a couple of winter hanging baskets with pansies/ivy/cyclamen and so on.
Definitely make sure to punch or drill drainage holes, and factor in some budget for compost (I tend to use Aldi's). I cheat and half to two-thirds fill my pots with garden soil, topping up with compost. I'm trying water-retaining crystals for the first time this year (from Poundland) to see if it makes a difference to watering. You need to be fairly dedicated to watering during the summer as pots dry out ridiculously quickly.
There are all sorts of weird and wonderful ideas on Pinterest for container gardening. A children's nursery near here has loads of wellies fixed to the wall filled with flowers
I tend to prefer bigger containers mind as the small ones dry out so fast (in the SE). I've also had to use the dreaded slug pellets to get my plants off to a good start 