Figures of ethos
Although it is certain that nearly every figure of speech may be employed in such a way as to promote the authority and credibility of the speaker (the appeal to ethos), many figures are specifically designed to do so, or else are likely to build the speaker's ethos in addition to any other effects.
Figures used to establish credibility (ethos)
This involves using learned quotes from memory or in your introduction, to give the positive impression that you are well educated and therefore credible as an advocate.
The Ad Herennium author suggests litotes (using understatement) as a means of expressing modesty in order to gain the audience's favor. If you read opening speeches by Curran or Erskine you will commonly find this technique being used.
You can find further details regarding anamnesis and litotes, which are particlarly useful in an opening speech and commonly found there, by clicking on the links below.
Figures that can damage credibility (ethos)
Stylistic vices, of course, will damage ethos such as cacozelia (see below), which is where a speaker will use language in an affected way, such as throwing in foreign words to appear learned.(Del Boy saying "Mon Dieu, Mon Dieu for example would be cacozelia) Cacozelia also involves having bad taste in words or selection of metaphor, so that rather than pleasing your audience you put them off.
And some figures seem more obviously artificial or affected, and can thus hurt one's credibility, especially if overused, such as paronomasia(punning).