Focussing is about placing your attention where it needs to be. There are a few simple tricks that can help you focus at critical moments; this page lists some of them

Find out about:
Using cue words
Cue actions
Visual focussing with a cute live demonstration
Using a shot routine

Cue words
Cue words are just words or phrases that trigger a response. They are useful in concentration management because they can focus you on the right thing at the right time; mnemonics, if you like. Some examples I've encountered are listed below for different shot phases. Notice that the largest number are concentrated during that most critical few seconds.
Preparation "Relax"; "Tall" (cue for stance);
Predraw "Control"; "Balance"; "Relax"
Draw/Aim/Shot "Power"; "Line"; "Smooth"; "Flow"; "Steady"; "Move!" (you don't get through a clicker by pulling, you get through by moving!); "Slide" (elbow along the aim line); "Burn" (... a hole in the gold); "Pressure" (cue to control hand and finger pressure); "Easy" ... and many others.
Cue words are personal - your own. You need to work out what points are most critical to you, and what you need to focus on at those points. Use cue words in practice first, to build a connection between word and action; then you can move into competition.
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Cue actions
Like cue words, actions can help focus attention. It is also reported that combined word/action cues work better than words alone. Actions needn't be irrelevant either; in archery, where actions depend only on the archer, it is easy to pick actions that fit well into every shot routine. Some examples (nowhere near a comprehensive list):
Carefully placing feet on line Lifting and settling shoulders before draw
Settling hand onto grip Squeeze and relax grip
Deliberate breath before draw Placing sight on gold
Breathing during shot    

Notice that most of these are part of possible 'shot routines' anyway. The trick to using them as cues is to associate them with something more specific; for example, "settling hand on grip" can be a reminder to focus on bow hand feel through the shot; a breath can trigger the right level of relaxation, and so on.
As with cue words, learn the right association in practice by establishing a little routine. When you are practising that relaxation during the shot, preface each shot with your cue action. When you want to focus on centre, practice focussing centrally as you place the sight. With a little practice, the action and the response work together to reinforce your focus on that critical element of your shooting.
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Quotable quote
This example of cue word use comes from Roy Matthews' book, Archery in Earnest
I may teach myself the individual parts, the actions, that make up the shot, by describing the detail in sentences... But when I made the shot there was no time for sentences, the arrow had to go. ...there was only time for words. Words that described how I felt.
...If I draw a line vertically through the gold, that is the line of the shot. If I draw a line horizontally through the gold, that is the height of the shot. Height is a non-active word. The word which meant something to me was power....
[The result was seeing the shot] in terms of LINE - POWER - RELAXATION
Roy Matthews, MBE, was a long-time British Champion, Master and Grand Master Bowman, and was still making international scores well into his sixties.
Visual focussing
This is more a way of bringing your focus in to a point; progressively withdrawing from peripheral events to end up on the gold. The exercise works like this:
Look around and beyond the target, accepting what you see
Look at 12 o'clock on the white, then track around the white and back to 12 o'clock
Move in to 12 o'clock on the black, then track around and back to 12 o'clock
Move in to 12 o'clock on the blue .. and around to 12 o'clock again
... and on into the red, and around
.. and into the centre of the gold, and let your focus stay there for a few seconds..
... and relax.

With a few run-throughs, you can probably compress this into a smooth transfer of focus from the environment, onto the target centre (or whatever you want to focus on) in a few seconds, with your attention 'peaking' for the critical phases of your shot. With a little more practice, that can happen during the prep and draw phases, so you shift your attention smoothly from 'building' the shot and into the aim/execution in time with your own shooting rhythm.
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Watch it happen
Press Go to see a (quick!) visual focussing exercise as it happens.
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