Re: How to get more impressions for specific keywords?
Impression share is based on ad rank (bid & quality score) and your campaign's budget. If your campaign isn't spending the daily budget, you can concentrate on the ad rank part.
Increasing quality score requires proper theming (grouping related keywords in an ad group), writing relevant ads, pointing to the most relevant landing page and using proper negatives. I'd recommend reading this thread for more suggestions on increasing Quality Score.
Next would bidding - depends on the vertical and competition. You can initially use Automatic bidding to get clicks and improve CTR, then move to a bidding method based on either conversion data or average position of ads.
Hope this helps!