Some routines are easier to practice than others. For example a silent manipulative act is pretty easy to practice. You setup, start your music and, go for it.
On ther other hand a standup act using audience participation is really difficult.
Simulating the spectator assistant is a key to practice. Often they hold the items, select things, and answer questions.
What have others done to simulate the spectator when you don't have someone available to act as one during practice?

61magic,
There is no really good way to simulate actual performing conditions. You might place chairs arranged as they would be for an audience in your venue and rehearse to the empty chairs. Performing for empty chairs is not very enlightening, but it can help with movement, blocking, etc. With performance experience, you can anticipate how new routines will play based on your past experience with other similar tricks. The only real way is to audience test new routines.

Carefully thought-out questions are just fine, but be careful and avoid asking questions which might get an answer you don’t want. I.e., the hackneyed question, “Did you see it go?” might be answered, “It’s in your other hand.” (Which “it” may be.)

Regards,
Gerald

The ideal is called a "test audience". For this you should use what I call "Associates", people who are neither magicians, nor laymen. In a perfect world, "associates" wouldn't exist, but they do. They are people, who are not, nor do they intend to be, magicians, but because they are closely associated with a magician or magicians, they have learned some of our concepts and theories, but have a complete respect for what we do. Put together test shows, let your audience know what it is, and tell them to act naturally and enjoy the show, but that there will be stops for stage direction work.

JackScratch,
Great idea! You've filled the chairs.

Regards,
Gerald

This is where your local magic club comes in handy! Take your routine down to the next meeting and try it out. Now I realize the audience are not "real" people (they're Magicians) but at least this way you'll be able to work out and blocking problems you have.

And you'll get an idea of how the routine flows, what props need to be where and such.

When it comes time to actually do the routine in your show for the first time, I'd advise putting it in between 2 of your stronger routines. That way if it doesn't fly too well, it won't seem too bad.

Very True Fred,
Good advice.
Clubs like the IBM and SAM are fantastic for this very thing.
Practice with magic friends first.
Practice makes perfect.
To any new magicians just starting out, seek a local club in your area.
The company alone does wonders.