I wouldn't be too stuck on the "salesman" term here. Engineering is (supposed to be) a (mostly) rational enterprise (sometimes). So the general approach would be to identify and understand technical and business related aspects/concerns and address them rationally.
Stabbing comfortably in the dark about your specific issue, I would say it is probably worth thinking about whether the most important aspects and impacts of your idea are clear in your peers' minds. Things like money, time/resourcing, potential to win projects could be considerations you'd want to address. The other answers provide already some good comments.
I'd like to add the following that I think hasn't been mentioned yet:
Timing in regards to impacted disciplines
A good idea / innovation may struggle if it means immediate, additional and potentially unpaid work for a partner discipline or colleague. E.g. (coming from a building design and construction background) if the building service engineers have essentially finished their design and your new idea would lead to substantial changes to their services, you have a problem because you are asking them either to work for free or to enter variation negotiations with the client. Same thing would happen no matter what stage they are if they have overclocked their hours for whatever reason.
Possible solutions could include:
- Push for your idea earlier in the project to give your colleagues time to digest and understand the impact of your idea
- Make your idea part of your team's bid (your idea might win the next project for the team)
- Is there an opportunity to influence the project brief?
- Consider that it might not happen in this project. Take your time to explain the idea to colleagues outside the context of the immediate project. Listen, ask questions, try to understand the reasons for the resistance (there might or might not be good reasons as already discussed by @TomAu and @jhabbot).
Resourcing impacts of partner disciplines
If your partner disciplines don't understand your idea, they might decide not to support it since they will have a hard time identifying how much resources and time they should allocate. Project time is often not the right time to fix this; people chase deadlines then (although sometimes project time is the only opportunity). Try to introduce ideas early on. Listen to your peers' concerns and offer help overcoming them.
Understanding the design process within your context
Who pays for the project? Who benefits from the specific idea? Who is impacted during design, execution, operation, phase out? Are there implications to the project program or budget (for clients and/or any disciplines involved) if your idea is taken up? Is special resourcing required? Does it make someone obsolete?
Don't get stuck on your one idea and on this one colleague who keeps telling you to improve your salesmanship
If this particular idea doesn't fly today, let it go for now, wait for the next one, however keep the idea at the back of your head and mention it again when the time comes.
If you can't convince this one key person, find another one or wait for another one. In the meantime keep asking what they mean by "salesmanship", if they have a good example for it, what they would have expected from you, if they could get you a mentor or if they could mentor you themselves. Their response to these questions might also help you understand their motivation and thinking.