do you explain to the customer exactly what your price includes? if you describe each step of the process, perhaps contrasting what you do with what lower priced folks can be expected to do, the customer may find your quote more palatable. i have found that most folks don't really understand how much time and effort--and expertise--is involved in a pressure washing job. i explain that wood requires special care to avoid splintering or furring, that reduced pressure to protect the wood requires more time to clean than, say, a cement driveway, that the cleaning will include application of a cleaning mixture, thorough pressure washing, and the application of a neutralizer/brightener. i also explain that expertise is required in selecting the appropriate chemicals, appropriate dwell time, and appropriate pressure. ditto with the staining/sealing process. i encourage potential customers to get other estimates but to find out what each bidder intends to do and how he or she intends to do it, in order to make a fair comparison. i also tell them what they cannot expect(that stain won't come up without tearing up the wood, and anyone who tells you different is exaggerating). you might also consider a satisfaction guarantee, but only after making sure the customer understands what can and cannot be expected.
consider also breaking your bid down into componants--so much for the stripping/cleaning, so much for the staining/sealing. also, the spindles of many decks are in good shape, and i'll give a quote that expressly excludes the spindles.
finally, in face to face situations where i can gauge the reaction to the price, i start out at the high end of the appropriate range but then reduce the amount by eliminating work that the customer probably didn't have in mind in the first place(if i don't do the curb, i can do it for $25 less) or by setting conditions that would justify a lower price(if i can do it on monday, when i'll be finishing up another job in this area, i can do it for $50 less). i've also had success by calling an undecided customer back and saying i finished a job early and don't have anything scheduled tomorrow, i'll do your job for $50 less to avoid having a non-productive day.
those approaches give you the possibility of your prefered charge but also let you back off of it without making the customer feel that you were high-balling on the first quote. any legitimate reasons for reducing an initial bid may do the trick(i'll do it for less to get an example of my work into this neighborhood, or if you are willing to be named a a referrence, or if you don't mind if i use before and after pictures of your deck to promote my business, and so forth.
all that said, i think the most important aspect to emphasize is that more work is required than they realize. i've had 2 customers in the last month who negotiated me down from my initial qute but who, when they saw how much work was required, and how conscienciously i performed the work, ended up paying me the amount i originally quoted, without me saying a word.
good luck
richard