There's lots said about pledge levels and stretch goals and I'm a long way from having all the answers, but I do have a few opinions. In no particular order this is my best advice in relation to pledge levels:
Stick It All Up: By the time you launch you'll have stacks of prototypes, original concept art, proof copies and more. Every bit of this is a collectible one off element of your game, put it on as a pledge level and have it contribute again to the project.
Cater To The Whales: Angel investors or Whales, what ever you want to call them; there are fantastic people who will want to invest in your project way beyond its base or even premium level because they believe in it. Give them an exclusive higher cost pledge level, which your prototype or proof copy is ideal for.
Find Your Extras: Figure out something extra you can do, you might offer a live play through of the game, even if only by Skype with a backer. At the very least if you can afford a gold Sharpie you can offer a signed edition version of your game for a few pounds extra.
Graduate Your Pledges: It looks odd if your pledge levels go from £1 for a thank you to £20 for the game to £300 for your hand printed prototype. Personally, I think jumps should be limited to £10 or doubling the previous pledge level. Look around Kickstarter if your short of ideas here. If nothing else remember that Kickstarter allows only a single pledge level per person so if people want an extra copy of your game to give as a gift, put one in a game library at a convention or even sell one, they need you to put up 2 games for a double price pledge level. If you have a base level of around £20 consider a print and play but be careful of PnP for lower priced games.
Offer A Clear Premium Pledge Level: During our SSO campaign we had at least one backer drop out because we lacked a clear "Premium" level pledge that would provide everything non-limited edition that the campaign would make available. It seems that there can be many pledge levels, but each individual level should be extremely clearly set out.
£1 Pledge: Put on a £1 for a thank you pledge, its not just a foolish gimmick. Kickstarter has its own community of followers and supporters and the £1 pledge allows people with followers to support you, signalling in turn to their followers to look at your game. They could click the blank pledge window and fill in £1 but mostly they won't. The £1 in your pocket is nice, but the followers and supporters are the real prize. Remember backers with a Kickstarter presence may wish to support you but honestly can't store yet another game.
Deliver Up Front: If at all possible find some way of offering a pledge level with an included delivery cost and if not make delivery costs extremely clear up front.
As for stretch goals, we only reached our first two stretch goals so take my advice here as far from expert, but I have learnt a few things from the experience. There's sometimes confusion about stretch goals, hidden profits and economies of scale which I'd like to comment upon. Our manufacturer, like many willing to print for independents, does not offer reductions of scale. Manufacturers that do require print runs of at least 3 times the quantity before their price comes within range of manufacturers that offer an initial lower price. To gain any kind of economy of scale for such a print run therefore a Kickstarter will need to double its run. This would mean that if a base game is £6000 intended to print 1000 copies the first stretch goal would need to be at £10,000. Goals and stretch goals of more than £2000 apart are unlikely to encourage backers as realistically reachable.
There are two main areas stretch goals can come from. The first is that generally the first game you print costs £3000 the second closer to £5 meaning that once you have covered that initial cost of your first print run, if you continue to raise funds at the same rate you should eventually have funds available to offer something to your backers. However, this still requires a significantly greater level of backing than you initial goal. Set up costs and economy of scales aside, most stretch goals will either be elements that were calculated into the original print run and unlocked in stages to offer a sense of progression, or a publisher buying a thank you present for their backers. It is useful to have a few ideas of what stretch goals you want to do and price them up with your manufacturer before hand. Also remember that they will often add printing time to your project, pushing back your fulfillment.