what's the easiest way to put space between 2 side-by-side buttons in asp.net

create a divider class as follows:

.divider{
    width:5px;
    height:auto;
    display:inline-block;
}

Then attach this to a div between the two buttons

<div style="text-align: center"> 
    <asp:Button ID="btnSubmit" runat="server" Text="Submit" Width="89px" OnClick="btnSubmit_Click" />
    <div class="divider"/>
    <asp:Button ID="btnClear" runat="server" Text="Clear" Width="89px" OnClick="btnClear_Click" />
</div>

This is the best way as it avoids the box model, which can be a pain on older browsers, and doesn't add any extra characters that would be picked up by a screen reader, so it is better for readability.

It's good to have a number of these types of divs for certain scenarios (my most used one is vert5spacer, similar to this but puts a block div with height 5 and width auto for spacing out items in a form etc.

Add a space   between them (or more depending on your preference)

    <div style="text-align: center">         
        <asp:Button ID="btnSubmit" runat="server" Text="Submit" Width="89px" OnClick="btnSubmit_Click" />
        &nbsp;
        <asp:Button ID="btnClear" runat="server" Text="Clear" Width="89px" OnClick="btnClear_Click" />
    </div>

#btnClear{margin-left:100px;}

Or add a class to the buttons and have:

.yourClass{margin-left:100px;}

This achieves this - http://jsfiddle.net/QU93w/

    <style>
    .Button
    {
        margin:5px;
    }
    </style>

 <asp:Button ID="Button1" runat="server" Text="Button" CssClass="Button" />
 <asp:Button ID="Button2" runat="server" Text="Button" CssClass="Button"/>

Old post, but I'd say the cleanest approach would be to add a class to the surrounding div and a button class on each button so your CSS rule becomes useful for more use cases:

/* Added to highlight spacing */
.is-grouped {   
    display: inline-block;
    background-color: yellow;
}

.is-grouped > .button:not(:last-child) {
    margin-right: 10px;
}

Spacing shown in yellow<br><br>

<div class='is-grouped'>
    <button class='button'>Save</button>
    <button class='button'>Save As...</button>
    <button class='button'>Delete</button>
</div>